ci: build: set ccache-cache-key

Set ccache-cache-key to matrix.os so that the action holds and restores
caches effectively.

Signed-off-by: Fabio Baltieri <[email protected]>

Author: fabiobaltieri

.github/workflows/build.yml

@@ -29,6 +29,7 @@ jobs:
         with:
           app-path: example-application
           toolchains: arm-zephyr-eabi
+          ccache-cache-key: ${{ matrix.os }}
 
       - name: Build firmware
         working-directory: example-application