[fix] fix update-flake-dependencies failures

[scottisloud]

This PR:

• checks for the presence of a PR on the automation/update-flake-dependencies branch, rather than the previous logic which checked whether a branch of that name exists.
  • If a PR doesn't exist, it opens a PR.
• Updated the list of reviewers to be current and recent members of the Open Source team.

Background
update-flake-dependencies would only open a PR with the updated flake.lock file if a branch called automation/update-flake-dependencies did not exist. Since that branch was not reliably deleted, and hasn't been for years, the workflow would find the branch and thus not open a PR.

[scottisloud]

#639 suggests the workflow logic is now correct, however, the commits it produces aren't signed. Investigating.....

[scottisloud]

Ah, okay it looks like commit bafbf36 switched from a GITHUB_TOKEN to a PAT. However, commits by automations using a PAT are not signed, and since that swap, we've never had signed commits from this automation. (I assume the switch from a GH token to a PAT was intended to work around GH's suppression of on: pull_request workflow triggers when using a GH token).

A few ways forward that I'll raise internally.

[scottisloud]

This is blocked until a new GH app is approved, which would allow workloads to make signed commits and PRs.