github/workflows: Enable ccache for esp32 build.

CCaches are scoped per-job.

Uses https://github.com/hendrikmuhs/ccache-action to get desired behaviour
(updating the cache on each run).

Signed-off-by: Angus Gratton <angus@redyak.com.au>

.github/workflows/ports_esp32.yml

@@ -48,5 +48,10 @@ jobs:
       if: steps.cache_esp_idf.outputs.cache-hit != 'true'
       run: source tools/ci.sh && ci_esp32_idf_setup
 
+    - name: ccache
+      uses: hendrikmuhs/ccache-action@v1.2
+      with:
+        key: esp32-${{ matrix.ci_func }}
+
     - name: Build ci_${{matrix.ci_func }}
       run: source tools/ci.sh && ci_${{ matrix.ci_func }}