Github Actions: Install older macos protobuf (3.21) (#1131)

As the most recent version has an abseil dependency which leads to build errors as C++17 is not activated for the build.

.github/workflows/build-macos.yml

@@ -25,7 +25,7 @@ jobs:
         target: 'desktop'
 
     - name: Install Dependencies
-      run: brew install ninja doxygen graphviz protobuf hdf5 pkg-config
+      run: brew install ninja doxygen graphviz protobuf@21 hdf5 pkg-config
 
     - name: Install Cap’n Proto
       run: |
@@ -85,6 +85,7 @@ jobs:
         -DECAL_THIRDPARTY_BUILD_QWT=ON \
         -DECAL_THIRDPARTY_BUILD_YAML-CPP=ON \
         -DCMAKE_BUILD_TYPE=Release \
+        -DCMAKE_PREFIX_PATH=/usr/local/opt/protobuf@21 \
         -DCMAKE_CXX_STANDARD=14 \
         -DPython_FIND_STRATEGY=LOCATION \
         -DPython_FIND_REGISTRY=NEVER