check_branch.py fails on chained PRs

[achave11-ucsc]

[edit: @hannes-ucsc]

This is a regression from 64b1419 (Fix: GitHub CI workflow selects dev when building prod branch (#5428, #6823, PR #6818)).

[achave11-ucsc]

Spike to diagnose.

[nadove-ucsc]

The script expects that the PR's target branch is associated with a shared deployment, as configured in azul_shared_deployments:

'azul_shared_deployments': json.dumps({
    'develop': ['dev', 'sandbox', 'anvildev', 'anvilbox'],
    'prod': ['prod'],
    'anvilprod': ['anvilprod', 'hammerbox']
}),

When creating a chained PR, the ideal behavior would be for the script to select the deployment associated with the target branch of the base PR (if the base PR is also chained, repeat this process until a non-chained base PR is reached). I presume this is possible via the GitHub API, but it might be overkill to go about implementing it.

My impression is that most chained PR's eventually lead to develop. Promotions being chained would be very unusual; I'm not sure about hotfixes though. If my impression is correct, it might be acceptable to just assume that dev is the appropriate deployment when the target branch isn't recognized.

[achave11-ucsc]

@hannes-ucsc: "Agreed, we can assume that chained PRs always target develop, albeit indirectly."

[hannes-ucsc]

For demo, attempt to reproduce.