Fix `packs.get` action

[jpavlav]

Addresses #6225. This should allow users to specify a branch when using the packs.get command. This addresses the assumption in the code that the primary branch of a git repository corresponding to an action is master.

• Add branch parameter to get.yaml. Default to master to maintain current functionality/expectations.
• Update run in pack_mgmt.get_install.py. Now accepts branch param and uses an fstring to represent the in git rev list.

[jpavlav]

I can add tests here, if that is required. There are, however, not any existing tests for this action, so I thought it might not be. Fundamentally, the change here is very small and the action is very simple. Whomever reviews this, let me know if new tests are required.