Fix ccache saving on cache hit

When a cache with the key exists it is NOT saved even when changed.
Hence make the key unique to a commit (SHA) and restore the most recent
cache with a key prefixed by the part without the SHA.

.github/workflows/ci.yml

@@ -137,7 +137,8 @@ jobs:
         if: env.B2_USE_CCACHE
         with:
           path: ~/.ccache
-          key: ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}
+          key: ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}-${{github.sha}}
+          restore-keys: ${{matrix.os}}-${{matrix.container}}-${{matrix.compiler}}-
 
       - name: Fetch Boost.CI
         uses: actions/checkout@v3