doc: Extend Gitaly /tmp workaround to cover Git execution path

Gitaly 14.7 until 14.9 have a bug where some files which are required at runtime may be deleted by systemd-tmpfiles(1), which then renders the instance broken. This bug affects two parts of Gitaly with Git hooks and the Git execution path used for bundled Git.

We have recently documented a workaround for this bug by simply disallowing the cleanup of these directories. This workaround only applies to the hooks directory though, so it may still happen that the Git execution environment is pruned. The consequence is that Gitaly becomes unable to execute Git at all.

Fix this oversight by also excluding /tmp/gitaly-git-exec-path-* from stale file cleanup.

Extends !85329 (merged)