Also use branches named main in workflow

[cg2121]

Description

Since GitHub defaults to 'main' now, also include it in the workflow when building, so developers don't have to change branch names if they choose to use it.

Motivation and Context

Make things easier

How Has This Been Tested?

Ran workflow

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

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