ci: build genpolicy on darwin

genpolicy is a developer tool that should be usable on MacOS. Adding it
to the darwin CI job ensures that it can still be built after changes.

On an Apple M2, the output of `uname -m` is `arm64`, which is why a new
case is needed in the arch_to_* functions.

We're not going to cross-compile binaries on darwin, so don't install
any additional Rust targets.

Fixes: #11635

Signed-off-by: Markus Rudy <mr@edgeless.systems>

.github/workflows/darwin-tests.yaml

@@ -18,6 +18,14 @@ jobs:
     name: test
     runs-on: macos-latest
     steps:
+    - name: Install Protoc
+      run: |
+        f=$(mktemp)
+        curl -sSLo "$f" https://github.com/protocolbuffers/protobuf/releases/download/v28.2/protoc-28.2-osx-aarch_64.zip
+        mkdir -p "$HOME/.local"
+        unzip -d "$HOME/.local" "$f"
+        echo "$HOME/.local/bin" >> "${GITHUB_PATH}"
+
     - name: Checkout code
       uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       with:
@@ -28,5 +36,8 @@ jobs:
         ./tests/install_go.sh -f -p
         echo "/usr/local/go/bin" >> "${GITHUB_PATH}"
 
+    - name: Install Rust
+      run: ./tests/install_rust.sh
+
     - name: Build utils
       run: ./ci/darwin-test.sh