Restrict test cache restoration

There is currently no way to invalidate the cache, so master won't pass CI if a breaking change has been made to the binary data formats.

Fix this by only restoring from the same commit SHA.

.github/workflows/osrm-backend.yml

@@ -397,8 +397,6 @@ jobs:
       with:
         path: ${{github.workspace}}/test/cache
         key: test-${{ matrix.name }}-${{ github.sha }}
-        restore-keys: |
-          test-${{ matrix.name }}-
 
     - name: Prepare environment
       run: |