Use openssl's sha1 and sha256, optionally (#15472)

.github/workflows/linux.yml

@@ -49,6 +49,8 @@ jobs:
         env:
           CC: gcc-7
           CXX: g++-7
+          # Test fallback SHA implementations
+          CMAKE_FLAGS: '-DENABLE_OPENSSL=0'
 
       - name: Test
         run: |