Use Bot token for update commit

[nyarly]

Our policies on our repos require that our commits all be verified in order to be merged. To accomplish this for github actions that produce PRs, we use tibdex/github-app-token@v1 to generate a token from a Github App installed on the project (really, just configured - there's no custom software being run) and then create commits based on that token, which both produces verified commits and allows other actions to run on the resulting PRs.

We like this solution a lot, but we would need update-flake-lock to use the token for commits in order to use it in our workflow. Some time ago, I forked it to nyarly/update-flake-lock and we use that internally. I issued PR #56 with our fork, but that was some time ago, and this repo has switched to JavaScript actions in the interim, so I don't think that PR will land now.

I don't usually use Javascript in my own actions, so I'm not sure I can devote the time to produce a new PR, or to understanding the Determinate actions library as well, but I wanted to open the issue, since the change is ultimately not large.