Use default branch, not 'main' as fallback

[pwtyler]

  branch:
    description: The branch to use as the base for PRs and commit the changes back to.
    required: false
    default: 'main'

https://github.com/jazzsequence/action-validate-plugin-version/blob/main/action.yml#L24-L27

Input should not default to "main", but instead if not provided, should use the default branch (usually but not always main)

[jazzsequence]

How would we canonically determine the default branch? Is that something that GitHub actions knows? I defaulted to main because I wasn't sure how the action would know what the default branch of the requesting repository would be.

[pwtyler]

Hmm, this may actually be a bug with your current implementation too. ~assumptions here may be wrong~ but my guess is, when this runs on the default branch and PRs against the release branch, then we might be PR'ing more than we want.

Current Behavior

(have not asserted this is truly the case or not) Cron actions only work on the default branch

• action on main (development branch)
• WF creates new branch (from main)
• PR is opened against release with changes we don't want there yet

Expected Behavior

• action checks out $BRANCH if it is set before checking out update-tested-up-to-version-(DATE), remains on current (default) branch if it is not.
• action uses the --base flag if $BRANCH is set, otherwise does not include it.

EDIT: Yep https://github.com/pantheon-systems/plugin-pipeline-example/pull/88/files