Currently the logic of generating nixpkgs Hydra jobs is to walk through
the pkgs evaluated for system = "x86_64-linux", collect any derivations
and their meta.platforms values. However, that doesn't work for
packages whose meta.platforms doesn't include x86_64-linux, as just
evaluating their meta attribute raises an error so they get skipped
completely.
As a less-intrusive fix (i.e. anything than rewriting the current package
enumeration logic), allow passing `config.allowUnsupportedSystem = true`
to permit evaluating packages regardless of their platform and use that
in the package listing phase.
Fixes #25200
Only cosmetic changes are done otherwise.
Real refactoring is left for later.
There's a small slow-down on my machine:
$ time nix-env -qa -P >/dev/null
gets from ~2.8 to ~3.5 seconds (negligible change in RAM).
That's most likely caused by sharing less computation between different
mkDerivation calls, and I plan to improve that soon.
