Don't absolutize 'git+file:' in parseURLFlakeRef() (backport #12280)

[mergify]

Motivation

This is needed to unbreak the 2.25 branch due to the interaction between #12269 and #12266 which our CI didn't catch. On master, this is resolved as side-effect of #10089.

See also #12273 (comment).

Context

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This is an automatic backport of pull request #12280 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 938f0f4 has failed:

On branch mergify/bp/2.24-maintenance/pr-12280
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 938f0f4fd.
  (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/libflake/flake/flakeref.cc
	both modified:   tests/functional/flakes/flakes.sh

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

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