buildifier checks should use the json output rather than parsing stdout/stderr

As mentioned by @vladmos in #943, the integration should likely be updated to use `--mode=check --format=json` rather than parsing stdout/stderr of a buildifier run, that should ensure bugs because out output formatting changes don't happen again.

The issue points to the buildifier integration and the #943 discussion; start by locating where a buildifier run's stdout/stderr is parsed. Check how the --mode=check --format=json output should be consumed, then verify the CI check still reports formatting failures correctly without depending on output formatting.

Written by the indexing model from the issue text.