ofborg does not use `$attr.meta.maintainers` when `$attr` is in the commit message

[Sergei Trofimovich]

Example PR: NixOS/nixpkgs#276696

There:

1. nix_2_3 (or nixVersions.nix_2_3) was explicitly specified in commit message.
2. nix repl agrees $attr.meta.maintainers does exist: nix-repl> nixVersions.nix_2_3.meta.maintainers => [ { ... } { ... } ]
3. the PR does cause nixVersions.nix_2_3 rebuild

@lilyinstarlight says it's a side-effect of nixVersions.nix_2_3 defining most of it's relevant attributes in a file different from default.nix (common.nix). I suspect nix is not the only one doing it.

Proposal: extend ofborg to also handle direct $attr.meta.maintainers expand attributes right from the commit message (and possibly filter only on packages affected by the rebuild if reasonably feasible). I suspect that is what it does today to fetch tests and build the attribute.

[Sergei Trofimovich]

NixOS/nixpkgs#276892 is another case where just following commit message attribute might be more desirable.

It's a no-op change from rebuild standpoint but I still would prefer maintainers to review that kind of change.