When you rebuild, use --show-trace and scroll through the logs (I know…) to get info on the package it was trying to build. You will see some message like while evaluating derivation <packagename>.
Some filtering along these lines could be done, e.g.
nixos-rebuild build --show-trace 2>/dev/stdout | grep 'while evaluating derivation'
(Normally I’d suggest nix-tree --derivation or such, but I doubt it’d work here as this is an eval-time error.)
There aren’t many packages that depend on electron_27 in nixpkgs anyway, a simple ripgrep gives the following:
- logseq
- micropad
- pocket-casts
- whalebird