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

Example PR: https://github.com/NixOS/nixpkgs/pull/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.

Start by tracing ofborg's existing handling of attributes from commit messages, using PR 276696 and the nix repl result for nixVersions.nix_2_3.meta.maintainers as the reproduction. Determine how attributes defined outside default.nix are resolved, then verify that direct $attr.meta.maintainers expansion works for rebuilt packages without breaking existing test and build attribute handling.