Caching arm compiler for builds

.github/workflows/ci_cortex_m4.yml

@@ -29,9 +29,17 @@ jobs:
 #     - name: Install build-essential
 #       run: sudo apt-get install build-essential
 
+    - name: Cache arm-none-eabi-gcc tools
+      id: cache-arm-gcc
+      uses: actions/cache@v1
+      with:
+        path: $HOME/arm-none-eabi-gcc-9-2019-q4
+        key: ${{ runner.os }}-arm-gcc-9-2019-q4
+
     # Get the arm-non-eabi-gcc toolchain   
     - name: Install arm-none-eabi-gcc
       uses: fiam/arm-none-eabi-gcc@v1
+      if: steps.cache-arm-gcc.outputs.cache-hit != 'true'
       with:
           release: '9-2019-q4' # The arm-none-eabi-gcc release to use.
           directory: $HOME/arm-none-eabi-gcc-9-2019-q4