Builds checks a rule repository's workflow on the default branch, not the rule's branch#4124

Organization → Builds reads a repository's ployz-build.yml on its default branch only (checkGithubBuildWorkflow in dashboard/src/modules/github/github-build.server.ts), and its Add and Update links open the default branch. Under an automatic deploy rule, a push runs the file on the pushed branch, and an approved dispatch runs the file at the head of the rule's branch. So a rule on a branch other than the default, such as production, can show the check mark while that branch still holds an older file, and its push builds then fail.

Fix: for a repository a rule names, check (and link) the rule's branch, which Builds already knows from the rule.

Frequency 2 (rules on a branch other than the default), severity 5 (a broken flow with a workaround: commit the file to that branch). F×S 10, found in review of getployz/ployz#4117, so filed rather than fixed there.