Use named keys for github actions caches

Each job will have its own cache.
This will keep size of individual cache lower and increase hit rate by
not trying to overwrite the same cache over and over again.

Author: janisozaur

.github/workflows/build_flang.yml

@@ -40,6 +40,8 @@ jobs:
 
       - name: ccache
         uses: hendrikmuhs/ccache-action@v1
+        with:
+          key: ${{ matrix.cc }}-${{ matrix.version }}
 
       - if: matrix.cc == 'gcc' && matrix.version == '10'
         run: |