Add automated job to update flake.lock file with signed commits

[charlespierce]

Info

• Second attempt at an automated job to update flake.lock, after Add automated job to update flake.lock file #264
• The original job worked, but the PR created did not have signed commits, so it wasn't mergeable.
• This approach adds a GPG key and associates the commits with @1password-bot (which is already used in other repos within the org)

Changes

• The job is created the same way as the previous PR, however we now add the author, committer, and GPG key information using repo secrets.

Testing

• We discovered in the last PR that testing via a PR-triggered job did not work as expected, so we'll likely need some quick follow-up testing once this is merged.

[LuminaSapphira]

Hopefully this one works!

[charlespierce]

Success! #268 (and even merged)