fix(ci): Update existing flake.lock PR if it exists already

[mrjones2014]

Overview

If a flake.lock automated PR already exists and hasn't been merged by the time a new job runs to create a new PR, it updates the existing PR instead of trying to create a new PR and failing because the branch already exists.

Type of change

• Created a new plugin
• Improved an existing plugin
• Fixed a bug in an existing plugin
• Improved contributor utilities or experience

Related Issue(s)

• Resolves: CI: flake.lock update job should update the existing PR if an unmerged one exists #467

How To Test

Read the changed bash script and ensure it makes sense. You can also test out the logic locally on a test repo. The thinking in the first few lines (the first if block) is:

• If the branch already exists on remote, checkout and pull before trying to update flake.lock (this is to ensure we can push new changes, e.g. a commit exists on remote that we don't have locally)
• Otherwise, create the branch and push the empty branch to remote
• Then run nix flake update, which now runs on the latest version of the file from the branch
• Push the changes to the branch
  • If the branch already existed and had a PR for it, pushing new commits to the branch will update the existing PR

Changelog

Update flake.lock CI automation to update existing PR if there is one.