`lock_root` dereferenced `$HOME` unconditionally, so under `set -u` the script
died with "HOME: unbound variable" in any environment that starts without it —
even when `PLAYWRIGHT_RESOURCE_LOCK_DIR` made HOME irrelevant. Surfaced by
running the suite under vitest's node environment, where `process.env` is
patched and spreading it does not carry HOME through to the child.
Resolve the lock path in order (explicit override, XDG_CACHE_HOME, HOME) and
fail with a clear message only when none of them is set. The test now names
PATH and HOME explicitly instead of spreading process.env.
Tommaso Casaburi
1a7d756d89