This moves most assertions out of generate-files and into a check
derivation. This should allow the CI to finish, even when there are
issues.
This also properly tests efmls, which was only checked partially before.
rust-analyzer is not covered because the existing assertions relate more
to edge-cases not handled by the generation script than the result it
builds.
If we access the asserts outside the derivation arguments, and inside
the code that _creates_ the derivation then we observe IFD in `nix flake
show`.
This commit fixes it, and allows `nix flake show` to work!
Fixes#1154
