💚 Lock CI testing to PlatformIO 5.2.5

.github/workflows/test-builds.yml

 
     - name: Install PlatformIO
       run: |
-        pip install -U https://github.com/platformio/platformio-core/archive/develop.zip
+        pip install -U https://github.com/platformio/platformio-core/archive/v5.2.5.zip
         platformio update
 
     - name: Run ${{ matrix.test-platform }} Tests