Add PR status check that fails unless based on `master`

[bfops]

Description of Changes

CI change - block PRs from merging if not based on master

API and ABI breaking changes

No

Testing

In a test repo:

• [x] Check that PRs merge if based on master
  • https://github.com/clockworklabs/SpacetimeDB/pull/1184
• [x] Check that PRs don't merge if not based on master
  • https://github.com/clockworklabs/SpacetimeDB/pull/1185