Automate cargo +nightly fmt on push

[kazk]

Automate what @clux has been doing after merging PRs (#505 (comment)). Uses https://github.com/marketplace/actions/git-auto-commit to commit any changes after cargo +nightly fmt --all.

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It's also possible to do this when PRs are opened. But to handle PRs from forks, we need to use pull_request_target event and forks needs to have GitHub Actions enabled.

[clux]

Looks great. Thank you! Would move the whole rustfmt thing into "don't want to think about it" territory.

[clux]

nit: we can probably lift this down into the commit action, i believe.
may at least try to run rustfmt everywhere possible?

[kazk]

Changed to run this when pushed to any branch in your repo.

[kazk]

@clux The push after rustfmt failed because master is protected. https://github.com/clux/kube-rs/runs/2604434250#step:5:54
We should be able to change the checkout action to use your personal access token, and commit as you. See https://github.com/stefanzweifel/git-auto-commit-action#push-to-protected-branches
Or use force push.

[clux]

Ah, that makes sense. Thank you. Ok. I'll probably set up a PAT and fix that up next PR. But ugh, the permissions on these PATs are a bit scary.

[kazk]

Yeah :( It should be possible to create a PR automatically based on the changes. I can look into that.