populate-depot: Allow SteamLinuxRuntime_whatever/.ref to exist
pv-wrap creates this lock file for compatibility with very old (pre-2021) versions of the Steam Linux Runtime, which took this lock before unpacking scout or soldier from a tarball, because at the time the Steampipe CDN did not cope well with a large number of small files.
Fixes: 0f89e027 "populate-depot: Write a top-level mtree"