fix[tool]: fix combined_json output for CLI (#3901)

the output json would not be produced because Path does not have a json
serializer

there are actually tests for `combined_json`, but they test the
`compile_files` API directly, whereas the offending code is in the very
outer `_cli_helper()` function.

the best (long-term) way to test this might be to have a harness which
runs the vyper CLI directly from shell, but that is not explored here
to reduce scope.

vyper/cli/vyper_compile.py

@@ -65,6 +65,7 @@ def _parse_cli_args():
 
 def _cli_helper(f, output_formats, compiled):
     if output_formats == ("combined_json",):
+        compiled = {str(path): v for (path, v) in compiled.items()}
         print(json.dumps(compiled), file=f)
         return