Use the same ccache cache key for all languages but not compilers

There doesn't seem to be any good reason to use per-language caches, as
there are files that are common to all languages.

But we do want to use different keys for the different compilers and
even different compiler versions, as they yield different results when
compiling the same code.

.github/workflows/ci.yml

@@ -63,7 +63,7 @@ jobs:
     - name: Install CCache
       uses: hendrikmuhs/ccache-action@v1
       with:
-        key: ${{ matrix.os || 'ubuntu-20.04' }}-${{ matrix.SWIGLANG }}
+        key: ${{ matrix.os || 'ubuntu-20.04' }}-${{ matrix.CC || 'gcc' }}${{ matrix.GCC }}
 
     - name: Configure
       run: |