[gh actions] Downgrade protobuf on macos (#1523)

* Downgrade protobuf on macos, as eCAL does not work with latest protobuf 3.26.

.github/workflows/build-macos.yml

@@ -25,7 +25,7 @@ jobs:
         target: 'desktop'
 
     - name: Install Dependencies
-      run: brew install ninja doxygen graphviz protobuf [email protected] pkg-config
+      run: brew install ninja doxygen graphviz protobuf@21 [email protected] pkg-config
 
     - name: Install Cap’n Proto
       run: |
@@ -85,7 +85,7 @@ jobs:
         -DECAL_THIRDPARTY_BUILD_QWT=ON \
         -DECAL_THIRDPARTY_BUILD_YAML-CPP=ON \
         -DCMAKE_BUILD_TYPE=Release \
-        -DCMAKE_PREFIX_PATH=/usr/local/opt/[email protected] \
+        -DCMAKE_PREFIX_PATH="/usr/local/opt/[email protected]:/usr/local/opt/protobuf@21" \
         -DCMAKE_CXX_STANDARD=17 \
         -DCMAKE_FIND_PACKAGE_PREFER_CONFIG=ON \
         -DPython_FIND_STRATEGY=LOCATION \