Proper platform handle gitcreds

[jt-metatheory]

Changes

• Adds support for a new ENV var of GIT_PRIVATE_TOKEN that mirrors the scripts in the dist folder for the main game.ci code to allow people to enter a git personal access token for Unity Package Manger Checkouts.

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [X] Documentation (updated or not needed)
• [X] Tests (added, updated or not needed)

[jt-metatheory]

Any thoughts on this? Happy to update as needed. It's working well for us but figured it's be nice for others having this issue.