Add workflow to push to aur on pushes to master

[jsjoeio]

Description

In order for us to complete the automated release pipeline from code-server release -> code-server-aur release, we need to way to automatically push changes to AUR.

Implementation Notes

I think this should be relatively straightforward from a GitHub Actions standpoint:

1. checkout repo
2. setup ssh keys/creds (example)
3. add new remote: git remote add aur ssh://aur@aur.archlinux.org/code-server.git
4. git push aur