ci(darwin): force-link brew formulas after cache restore

Symptom: `ccache: command not found` in the Configure ccache step on
runs that hit the brew cache.

Root cause: actions/cache restores /opt/homebrew/Cellar/<formula> but
NOT the bin symlinks at /opt/homebrew/bin/*. The subsequent
`brew install` sees the Cellar entries present and decides "already
installed" — without re-running the link step. So on cache-hit runs
none of the cached formulas are actually on PATH.

Fix: explicit `brew link --overwrite` for every formula we install,
right after `brew install`. --overwrite tolerates leftover symlinks
from a partial earlier install. The 2>/dev/null + || true keeps the
step from failing if a formula is already correctly linked.

Pre-existing flake; surfaces more often as Darwin matrix coverage
grows after the llama-cpp-darwin consolidation in #9731.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

.github/workflows/backend_build_darwin.yml

@@ -101,6 +101,12 @@ jobs:
           # the brew cache content stays stable across every backend in the
           # matrix — they all share one cache key.
           brew install protobuf grpc make protoc-gen-go protoc-gen-go-grpc libomp llvm ccache
+          # The brew cache restores the Cellar dirs but NOT the bin symlinks
+          # at /opt/homebrew/bin/*. brew install above sees the Cellar present
+          # and decides "already installed" without re-linking, so on a cache-
+          # hit run the formulas aren't on PATH. Force-link them; --overwrite
+          # tolerates pre-existing symlinks from earlier installs.
+          brew link --overwrite protobuf grpc make protoc-gen-go protoc-gen-go-grpc libomp llvm ccache 2>/dev/null || true
 
       - name: Save Homebrew cache
         if: github.event_name != 'pull_request' && steps.brew-cache.outputs.cache-hit != 'true'