Overlay losing buildDependencies

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I’m overriding src for libfprint to use a fork with experimental support for Chipsailing’s CS9711.

final: prev: {
  libfprint = prev.libfprint.overrideAttrs(old: {
    pname = old.pname + "-cs9711";
    src = prev.fetchFromGitHub {
      owner = "ddlsmurf";
      repo = "libfprint-CS9711";
      rev = "03ace5b20146eb01c77fb3ea63e1909984d6d377";
      hash = "sha256-gr3UvFB6D04he/9zawvQIuwfv0B7fEZb6BGiNAbLids=";
    };
  });
}

This leads to build failure:

       > Run-time dependency nss found: NO (tried pkgconfig and cmake)

nss however is a buildInput on the original package. I can add it back manually, and the build goes past that point, but then more missing dependencies show up

final: prev: {
  libfprint = prev.libfprint.overrideAttrs(old: {
    pname = old.pname + "-cs9711";
    src = prev.fetchFromGitHub {
      owner = "ddlsmurf";
      repo = "libfprint-CS9711";
      rev = "03ace5b20146eb01c77fb3ea63e1909984d6d377";
      hash = "sha256-gr3UvFB6D04he/9zawvQIuwfv0B7fEZb6BGiNAbLids=";
    };
    buildInputs = (old.buildInputs or []) ++ [ prev.nss ];
  });
}

Run-time dependency nss found: YES 3.101.1
Run-time dependency gudev-1.0 found: YES 238
Run-time dependency udev found: YES 257
meson.build:283: WARNING: udev hwdb will be installed by both systemd and libfprint
Dependency glib-2.0 found: YES 2.82.5 (cached)
Program /nix/store/gdq1i8iwvxjznbfnmg83lg5sgy35qp8i-glib-2.82.5-dev/bin/glib-mkenums found: YES (/nix/store/gdq1i8iwvxjznbfnmg83>
Dependency glib-2.0 found: YES 2.82.5 (cached)
Program /nix/store/gdq1i8iwvxjznbfnmg83lg5sgy35qp8i-glib-2.82.5-dev/bin/glib-mkenums found: YES (/nix/store/gdq1i8iwvxjznbfnmg83>
Dependency glib-2.0 found: YES 2.82.5 (cached)
Program /nix/store/gdq1i8iwvxjznbfnmg83lg5sgy35qp8i-glib-2.82.5-dev/bin/glib-mkenums found: YES (/nix/store/gdq1i8iwvxjznbfnmg83>
Dependency glib-2.0 found: YES 2.82.5 (cached)
Program /nix/store/gdq1i8iwvxjznbfnmg83lg5sgy35qp8i-glib-2.82.5-dev/bin/glib-mkenums found: YES (/nix/store/gdq1i8iwvxjznbfnmg83>
Configuring fpi-drivers.c with command
Did not find CMake 'cmake'
Found CMake: NO
Run-time dependency opencv4 found: NO (tried pkgconfig)

libfprint/sigfm/meson.build:4:9: ERROR: Dependency "opencv4" not found, tried pkgconfig

What the heck is going on? Where did the original buildDependencies go?