ci: fall back to /tmp for the uv install lock when TMPDIR is unwritable

Addresses Claude Code Review on #1630:

- flock hard-fails (and skips running uv pip install entirely) if its
  lock file's directory doesn't exist or isn't writable. TMPDIR can be
  stale on HPC login nodes (e.g. left over from a prior job's
  since-deleted scratch dir), so an unconditional flock target risked
  breaking installs that worked fine before this lock existed. Guard
  with the same -w check already used for the UV_CACHE_DIR redirect,
  falling back to /tmp.

- Clarified the comment: uv's cache is shared per-user by default
  (~/.cache/uv), not just in the CI node-local-redirect case, so the
  serialization also protects concurrent local builds, not only
  self-hosted CI matrix legs.

Author: sbryngelson

toolchain/bootstrap/python.sh

@@ -189,16 +189,23 @@ if ! cmp "$(pwd)/toolchain/pyproject.toml" "$(pwd)/build/pyproject.toml" > /dev/
             export UV_CACHE_DIR="${TMPDIR:-/tmp}/uv-cache-${USER:-$(id -un)}"
         fi
 
-        # Self-hosted HPC runners (Frontier, Phoenix) run several matrix legs
-        # (interfaces/shards) as the same OS user on the same login node at
-        # once, all sharing the UV_CACHE_DIR above. uv's own cache lock
-        # protects individual entries, but concurrent installs can still race
-        # while uv extracts/prunes the shared archive-v0 store, leaving a
-        # corrupted entry (e.g. a missing dist-info METADATA file) that fails
-        # every subsequent install until the cache is manually cleared.
-        # Serialize the install call itself so only one uv process touches a
-        # given cache dir at a time.
-        UV_INSTALL_LOCK="${TMPDIR:-/tmp}/mfc-uv-install-${USER:-$(id -un)}.lock"
+        # uv's cache (~/.cache/uv by default, or the node-local redirect above
+        # in CI) is shared per-user across every repo/worktree/matrix leg. uv's
+        # own cache lock protects individual entries, but concurrent installs
+        # from separate uv processes can still race while one extracts/prunes
+        # the shared archive-v0 store, leaving a corrupted entry (e.g. a
+        # missing dist-info METADATA file) that fails every subsequent install
+        # until the cache is manually cleared -- both across self-hosted CI
+        # matrix legs (Frontier, Phoenix) sharing a login node, and across
+        # concurrent local builds by the same user. Serialize the install
+        # call itself so only one uv process touches a given cache dir at a
+        # time. Fall back to /tmp for the lock file itself if TMPDIR isn't
+        # writable (e.g. a stale TMPDIR left over from a prior job's
+        # since-deleted scratch dir), so a bad TMPDIR can't break installs
+        # that used to work fine before this lock existed.
+        UV_LOCK_DIR="${TMPDIR:-/tmp}"
+        [ -w "$UV_LOCK_DIR" ] || UV_LOCK_DIR=/tmp
+        UV_INSTALL_LOCK="${UV_LOCK_DIR}/mfc-uv-install-${USER:-$(id -un)}.lock"
         if command -v flock > /dev/null 2>&1; then
             uv_install() { flock "$UV_INSTALL_LOCK" uv pip install "$@"; }
         else