CI: Fix target branch for push action

[PatTheMav]

Description

Fixes GitHub Actions workflow for publishing new formulas.

Motivation and Context

The repository uses the main variant instead of the more common master variant.

How Has This Been Tested?

Will need to be tested by doing a publish run on an open formula update.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.