Added CI to create PRs to bump to latest Ark timestamps

[wtripp180901]

Added a nightly workflow which will check the URLs defined in appliances_repo_timestamp_sources for newer timestamps, update appliances_repo_timestamps and create a PR if any timestamps have been updated

[sjpb]

See discussion on slack re. simplifying things so we're not defining repo URLs in 3x places.

Also needs to
a) kick off an image build in the PR - and not run the stackhpc.yml workflow

• Could do this by changing that workflow to not run on branch names like e.g. auto/ and setting that branch for the PR
  b) make a commit to the PR to change environments/.stackhpc/terraform/cluster* vars to the new image - then run the stackhpc workflow

OR

• do the image build on the branch
• update ../cluster* definition
• THEN open the PR (which runs stackhpc.yml)

[wtripp180901]

todo: test fatimage workflow dispatch still works (will affect the auto/bump-timestamps branch too if it doesn't)

[wtripp180901]

looks like input.event_name is inherited from caller and is running image build/ci on this branch so needs reworking