Setup GitHub App to be able to trigger `on: push` and `on: pull_request` workflows for auto-pr

[PLeVasseur]

I posted this on Zulip:

Okay, I added the tag, but I think it's possible that due to how GitHub works, we won't see the build triggered by the create-pull-request action with how the workflow is currently set up.

Here's what the create-pull-request docs say:

If you want pull requests created by this action to trigger an on: push or on: pull_request workflow then you cannot use the default GITHUB_TOKEN. See the documentation here for further details.
I think that's why the build action is not firing on the PR and it's left in this pending state forever

and

Reading further, it seems we need to use another type of token. I could certainly create a Personal Access Token, but then that ties it to me. I'm wondering if I could get a bot account created which is associated to the rustfoundation org, then use that bot account to generate a PAT.

(This is how I've created a PAT within a company in the past, we had a bot account separate from any individual's)

Other token options:

• Classic Personal Access Token (PAT) with repo scope.
• Fine-grained Personal Access Token (PAT) with contents: write and pull-requests: write scopes.
• GitHub App tokens with contents: write and pull-requests: write scopes.

To which Pietro replied:

you should create a github app
https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Running.20workflows.20after.20a.20bot.20pushes.20a.20commit.20to.20a.20PR

So I need to figure out how to do this and whether it's something I can do on my own or require support from someone from the rustfoundation org to pull off ☺️

[PLeVasseur]

Closed via #167

Confirmed in #169

Build failed, but that's fine, we just wanted the build step to actually run, which it now does!

[x0rw]

This looks great 🔥🔥🔥