local-derivation-goal: improve "illegal reference" error (backport #12105)

[mergify]

Motivation

Before the change "illegal reference" was hard to interpret as it did not mention what derivation actually hits it.

Today's nixpkgs example:

Before the change:

$ nix build --no-link -f. postgresql_14
...
error: derivation contains an illegal reference specifier 'man'

After the change:

$ nix build --no-link -f. postgresql_14
...
error: derivation '/nix/store/bxp6g57limvwiga61vdlyvhy7i8rp6wd-postgresql-14.15.drv' output 'lib' contains an illegal reference specifier 'man', expected store path or output name (one of [debug, dev, doc, lib, out])

Context

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This is an automatic backport of pull request #12105 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f3dbaa3 has failed:

On branch mergify/bp/2.24-maintenance/pr-12105
Your branch is ahead of 'origin/2.24-maintenance' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit f3dbaa3f5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   src/libutil-tests/strings.cc
	both modified:   src/libutil/strings.hh

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally