use shasum for macos

Author: chirag-parmar

.github/workflows/nimbus_verified_proxy.yml

@@ -90,7 +90,7 @@ jobs:
           build/nimbus_verified_proxy --help
           # "-static" option will not work for osx unless static system libraries are provided
           make ${DEFAULT_MAKE_FLAGS} nimbus-verified-proxy-test
-          sha512sum "build/nimbus_verified_proxy" > "nimbus_verified_proxy.sha512sum"
+          shasum -a 512 build/nimbus_verified_proxy > "nimbus_verified_proxy.sha512sum"
 
       - name: Upload release binary artifacts
         if: success() && startsWith(github.ref, 'refs/tags/')