GitHub CI workflow should fail if cargo fmt edits source code

[KendallWeihe]

Prior to #187, if the call to cargo fmt actually changed the underlying source code then the pipeline would fail, which would indicate to the developer they need to format their code and then push changes. But since we moved to #187 that feature is no longer enforced, I noticed because in #199 I had formatting changes which were from where #193 didn't run just lint prior to merging, but it wasn't caught because the pipeline didn't fail. The pipeline shouldn't allow this to happen.