fix CI (hopefully)

.github/actions/prepare-protobuf-cache/action.yml

@@ -0,0 +1,45 @@
+name: Prepare Protobuf Cache
+description: Restore or build the cached protobuf installation.
+
+inputs:
+  protobuf-commit:
+    description: Protobuf tag or commit to build.
+    required: true
+  mold-linker-flags:
+    description: Linker flags used to force mold.
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Restore protobuf cache
+      id: restore-protobuf-cache
+      uses: actions/cache/restore@v4
+      with:
+        path: |
+          /usr/local/lib/libproto*
+          /usr/local/include/google/protobuf
+        key: protobuf-${{ runner.os }}-${{ inputs.protobuf-commit }}
+
+    - name: Install protobuf
+      if: steps.restore-protobuf-cache.outputs.cache-hit != 'true'
+      shell: bash
+      run: |
+        git clone --depth 1 --branch ${{ inputs.protobuf-commit }} https://github.com/protocolbuffers/protobuf
+        cmake -S protobuf -B protobuf/build -G Ninja \
+          -DCMAKE_BUILD_TYPE=Release \
+          -DCMAKE_EXE_LINKER_FLAGS="${{ inputs.mold-linker-flags }}" \
+          -DCMAKE_SHARED_LINKER_FLAGS="${{ inputs.mold-linker-flags }}" \
+          -DCMAKE_MODULE_LINKER_FLAGS="${{ inputs.mold-linker-flags }}"
+        cmake --build protobuf/build
+        sudo cmake --install protobuf/build
+        rm -rf protobuf
+
+    - name: Save protobuf cache
+      if: steps.restore-protobuf-cache.outputs.cache-hit != 'true'
+      uses: actions/cache/save@v4
+      with:
+        path: |
+          /usr/local/lib/libproto*
+          /usr/local/include/google/protobuf
+        key: protobuf-${{ runner.os }}-${{ inputs.protobuf-commit }}