branch as input for premerge workflow

[TrevorBenson]

Description

Adding the branch as a workflow input, and then setting it via input when defined allows pre-merge testing without opening a pull request.

Where should the reviewer start?

Original Workflow w/o Branch

1. Choose Pre-merge workflow under actions
2. Leave the alpha or main as the branch in theUse workflow from dropdown.
3. Click Run workflow
4. Observe BRANCH is missing and workflow eventially fails.

Updated Workflow w/ Branch

1. Choose Pre-merge workflow under actions
2. Select the branch-as-workflow-input as the branch in theUse workflow from dropdown.
3. Click Run workflow
4. Observe BRANCH is populated and the workflow runs as expected.

Motivation and context

• Remove the need for opening a PR to test pre-merge workflows.
• Reduce noise to collaborators emails who are monitoring the repository for Pull Requests.
• Simplified Fork testing, commits to guild-deploy.sh or cabal-build-all.sh are no longer required to test a premerge workflow.

Which issue it fixes?

None, just feature/improvement.

How has this been tested?

With a mix of other enhancements (cherry-picked into separate PR's to submit) it was tested on this forked repo workflow