Clarify that --keep-failed with remote builders will keep the failed build directory on that builder (backport #13320)

[mergify]

Motivation

While some may be able to intuit that passing --keep-failed when using remote builders will indeed keep the failed build directory on the remote builder, others may expect it to be accessible on the local machine (especially when the build log includes the message note: keeping build directory '/tmp/nix-shell.8G497s/nix-build-failing.drv-16/build' for example).

This adds a warning in the case that a remote build fails and --keep-failed was passed, to inform the user that the failed build directory was kept -- but on the remote builder itself.

Context

Related: DeterminateSystems#85

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This is an automatic backport of pull request #13320 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0712339 has failed:

On branch mergify/bp/2.29-maintenance/pr-13320
Your branch is up to date with 'origin/2.29-maintenance'.

You are currently cherry-picking commit 071233991.
  (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)

Changes to be committed:
	modified:   tests/functional/build-remote.sh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/build-remote/build-remote.cc

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