Profile-scoped lock used when the installation locks directory is unwritable.
A system or root-owned sys.prefix makes UPDATE_LOCK_FILE's parent
uncreatable for a normal user. Without this fallback the lock would fail open
on every launch, which is the concurrent-install race it exists to prevent.
Profile-scoped serialization is weaker than installation-scoped — two profiles
sharing one unwritable installation would not serialize against each other —
but it is strictly better than no lock at all.
