CI: install protoc for building temporalio from sdk-python git source

Author: DABH

.github/workflows/ci.yml

@@ -33,6 +33,13 @@ jobs:
       - uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6
         with:
           python-version: ${{ matrix.python }}
+      # protoc is required to build temporalio from the sdk-python git source
+      # (see [tool.uv.sources] in pyproject.toml). Drop this step once the SDK
+      # ships a release that includes the LangGraph plugin.
+      - uses: arduino/setup-protoc@c65c819552d16ad3c9b72d9dfd5ba5237b9c906b # v3
+        with:
+          version: "23.x"
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
       - run: uv tool install poethepoet
       - run: uv sync --group=dsl --group=encryption --group=trio-async
       - run: poe lint