How to actually override nixpkgs.inputs for e.g. emacs-overlay so it hits cache?

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in my flake.nix

    # https://hydra.nix-community.org/build/16627869#tabs-buildinputs
    emacs-overlay-unstable-git-native-comp = {
      type = "github";
      owner = "nix-community";
      repo = "emacs-overlay";
      rev = "560bb95b7831ae6ad643e41f1281af1dc595c045";
      inputs.nixpkgs = {
        type = "github";
        owner = "nixos";
        repo = "nixpkgs";
        rev = "5a0e0d73b944157328d54c4ded1cf2f0146a86a5";
      };
    };

so this is supposed to override inputs.nixpkgs with 5a0e0d73b944157328d54c4ded1cf2f0146a86a5, right? those are indeed what ends up in the flake.lock.

    "emacs-overlay-unstable-git-native-comp": {
      "inputs": {
        "flake-utils": "flake-utils_8",
        "nixpkgs": "nixpkgs_6"
      },
      "locked": {
        "lastModified": 1658859882,
        "narHash": "sha256-y15h05wrN+BWifRhBjTno3VpVmD0ditH3NwFW2j8UCA=",
        "owner": "nix-community",
        "repo": "emacs-overlay",
        "rev": "560bb95b7831ae6ad643e41f1281af1dc595c045",
        "type": "github"
      },

    "nixpkgs_6": {
      "locked": {
        "lastModified": 1658737577,
        "narHash": "sha256-xosJ5nJT9HX+b6UWsSX6R+ap4AdZOCrl/r+IKFp2ASQ=",
        "owner": "nixos",
        "repo": "nixpkgs",
        "rev": "5a0e0d73b944157328d54c4ded1cf2f0146a86a5",
        "type": "github"
      },
      "original": {
        "id": "nixpkgs",
        "type": "indirect"
      }
    },

but when try rebuild my system, that won’t hit nix-community cache (i removed ~/.cache/nix to make sure) and starts building emacs from source even though nix community has the cache for that and i can download the nar directly just fine.

StorePath: /nix/store/zpns283fhk877syvgjh1skfdc2j967bk-emacs-git-native-comp-20220726.0
URL: nar/b52b5e95c22b1a75414ee6e91d7446634dbe7cff51b5b58365156d5f833ceede.nar.xz
Compression: xz
FileHash: sha256:b52b5e95c22b1a75414ee6e91d7446634dbe7cff51b5b58365156d5f833ceede
FileSize: 79420840
NarHash: sha256:04c76qdg4ni08j2m6m4ypmaymm5qjc23027ylrhknxnn06lgkwi7
NarSize: 313634840
References: 0figld4yz6k9bk30w05wxdybqq404y8j-xcb-util-0.4.0 0gzd0049bywqhzi5anbydsxil7r1lfrj-binutils-2.38 1ayz7sydm1rbkfspzy6qq5ih21d1mi3z-gnutls-3.7.6 1rsj0mw8vqlnhk59wkw4i3yal4qw19k0-libXrender-0.9.10 2ggbc04aiq9zf09g49gz9hi3wgk1i4vk-harfbuzz-3.3.2 2wx0faw05ncvgaz5jihd5pda1mzw63mb-libselinux-3.3 38ychklifhkvfjzgqr1i97916cmiwdhj-glib-2.72.3 48s4h862snj1m8dcw58rdj6mpj04188k-systemd-250.4 4ay9q1d8j82p918r49jlzsixw1bpdhm0-libX11-1.7.2 7mcrxfhl2qxx5m6cf991jv8sg9bzb7v0-freetype-2.12.1 7yxmpp55m3n1m6x3gzyrws31i5janhal-librsvg-2.54.4 8ri08j9vhgslirg2p966l830wv3qvwiy-giflib-5.2.1 a2fyr6w9zp07a5615mzmcbqn0jg78hv0-m17n-lib-1.8.0 a7zkqrfyvd9xsr6mvjwqsjb0xqnydaad-jansson-2.14 arysd09ka49dpdai09b83ndkqnilgnyh-gsettings-desktop-schemas-42.0 c8p91c6fm67f4ldza6d811wfj61far9f-libxml2-2.9.14 dwiqy7ixa584rb60lxc8hd10bjryx2ww-libpng-apng-1.6.37 dy7i1aqsf0rfm91bcif0z5m9zg6clmd7-libxcb-1.14 fcyqa5s7j42gam1qkpkb10453d85zf7q-sqlite-3.39.0 ffjvj65in9nf0jnlnjw03wbqmcq59k1c-gmp-with-cxx-6.2.1 jfqvyzcfxfgbknz45ywxv890s7hchydp-binutils-wrapper-2.38 jws83dlza8fl5w9mvk5zjaj7mmcknaa3-gtk+3-3.24.34 k3421p8w6f6pgpcmrminbw1sznrdrvkl-libSM-1.2.3 kiwdwd6zw4svi9jlr95yg1p5pgpjxn1v-gcc-11.3.0 l9lbrnhrswxy2wvjl09gmg2mry7dg0xp-libXpm-3.5.13 mg0qwr5c954qx4izf9yxahv6c93c5qgf-libXfixes-6.0.0 mhhlymrg2m70r8h94cwhv2d7a0c8l7g6-glibc-2.34-210 npcz8z6zl27lzpy0b22isd6kb4ya2llj-fontconfig-2.14.0-lib nsicd68siz5nc44q8zz5cpb1v764bvfa-libotf-0.9.16 p2mxc5x4h4578f4l0948h98xl3yqdjxj-libjpeg-turbo-2.1.3 pkv4ngqc7arfqpr1bsn1bappvms9xbfg-ncurses-6.3-p20220507 q8rdgv8x3hk9ip1v0rraywf0f8bi37nc-libXinerama-1.1.4 qaz5hkfiw2pkfjz1721kbbxb6ks9z19z-libXi-1.8 qm2lv1gpbyn0rsfai40cbvj3h4gz69yc-bash-5.1-p16 rxzkc6x3786xbm1si93b0gj5hxx9zfck-libgccjit-11.3.0 slacsfxv01gjcaqjb9i5k6v77vf80w82-gettext-0.21 w25nadr7247krvg5nbxr9slnhq76dakc-zlib-1.2.12 wdnxn318gyp75k34f2cscfy6g1k82ybb-libXrandr-1.5.2 wffw18z76xs7r3bwv2qy44r0gshh4qnx-gdk-pixbuf-2.42.8 wmx6zyv93xh0mkkkzdl4iv2wqmk1pixb-libtiff-4.4.0 x0cpggi73rhsm6cwfl9r8q0qvzx72ysl-libXext-1.3.4 x9zh2nh0dqwisgmj7avnysk35xm45g2y-libICE-1.0.10 xgr8zb2sj6g30qfhk7p7mv0ygxck6z8i-cairo-1.16.0 xhdf882p0a25vxz2x94xifggfxqim4n7-atk-2.38.0 yhkn33x0ncx2zfk6a6dy8cdsjhs15hzg-dbus-1.14.0-lib ylnb8y7awrjhlxr1308rchahqvmykxj6-dconf-0.40.0-lib zg4x984qw3zaxn0xy6qgjzhrk1k3h9d4-libXcomposite-0.4.5 zpns283fhk877syvgjh1skfdc2j967bk-emacs-git-native-comp-20220726.0 zpvq7njsx28yjzfg4n97l83sm0gdhk4c-pango-1.50.7
Deriver: pqrwxsfl5yw5hycgz440lzbyvgb9nq3m-emacs-git-native-comp-20220726.0.drv
Sig: nix-community.cachix.org-1:23IcF2rhqDiKsBFkSYY9vp76OQnYaczrde54DPHq1ys+urC1zIdk60czSNluWYqG+4Y1bPxqvaE1C//jf778Bg==

// GET https://nix-community.cachix.org/zpns283fhk877syvgjh1skfdc2j967bk.narinfo

it seems cache will hit only when i use 5a0e0d73b944157328d54c4ded1cf2f0146a86a5 for my top level inputs.nixpkgs. am i missing something?

for more context https://github.com/nix-community/emacs-overlay/issues/122#issuecomment-1194839690