create cache entry for paths already in the nix store (backport #12911)

[mergify]

This allows path:/nix/store/* paths to not be copied twice to the nix store.

Motivation

Context

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This is an automatic backport of pull request #12911 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 61c6210 has failed:

On branch mergify/bp/2.26-maintenance/pr-12911
Your branch is up to date with 'origin/2.26-maintenance'.

You are currently cherry-picking commit 61c6210db.
  (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 <file>..." to mark resolution)
	both modified:   src/libfetchers/fetch-to-store.cc
	both modified:   src/libfetchers/fetch-to-store.hh
	both modified:   src/libfetchers/path.cc

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