ci: fix missing llvm on macOS (#116)

Author: gmusya

.github/workflows/build-n-test.yml

@@ -50,6 +50,9 @@ jobs:
     runs-on: macos-latest
     steps:
     - uses: actions/checkout@v4
+    - name: install brew deps
+      run: |
+        brew install llvm@17
     - name: fetch-deps
       run: |
         mkdir -p _build && cd _build