automate byte-buddy build

[hughsimpson]

The proposed workflow here is:

• Bump 'byte-buddy-tag' and open a pr
• The new acton kicks off, and builds a patched byte-buddy using the target tag in byte-buddy-tag
• The action commits, and pushes

I had to make the assumption that the pr is opened from a fork owned by the committer, because pull_request actions don't seem to 'know' the forked repo name.

I also don't know if the bot will actually have permissions to push to the branch... Although I've set them to

  Contents: write
  Metadata: read
  PullRequests: write

on the new action, these settings will not take effect until after the pr is merged, so the job is currently running with

 Contents: read
 Metadata: read
 PullRequests: read

-- these permissions are clearly not enough to push. I have kinda tested it against myself here and it seems to work, but whether it will in practice will depend on whether Allow edits and access to secrets by maintainers also grants the power to push to bots... To really ensure this worked, I think I'd need to fiddle about with branches on kanela... 🤔 Alas I have not those permissions...

Edit: Ok I got someone to test this flow for me by pr-ing to my branch, but the bot doesn't have permissions to push, so this approach won't work. Back to the drawing board...