[Github] Print diff in code format helper (#72742)

Currently, when the code format action fails, it leaves no log of the
diff in the output within the action itself. This has caused confusion
for some users of the action, especially when the comment becomes buried
in a 100+ comment review and someone isn't super familiar with the inner
workings of the CI. This patch prints the diff produced by the code
formatter to stdout so that it is viewable by clicking on the failed
action. This should have very little cost and make things slightly less
confusing for those that run into this situation.