nixos/gitlab: Fix state directory permissions
nixos/gitlab: Fix state directory permissions

Since the preStart script is no longer running in privileged mode, we
reassign the files in the state directory and its config subdirectory
to the user we're running as. This is done by splitting the preStart
script into a privileged and an unprivileged part where the privileged
part does the reassignment.

Also, delete the database.yml symlink if it exists, since we want to
create a real file in its place.

Fixes #68696.

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