CI - Run unreal tests

[bfops]

Description of Changes

Before merging this

1. uncomment the part of the workflow file that overrides GIT_REF. This was causing me issues during testing because it somehow kept pulling back in a rolled-back commit from the base branch, but I suspect that this will no longer be true once new commits are added to the base branch.
2. connect a proper GitHub account to an Epic Games account (specifically, an account that multiple people have access to, e.g. maybe all of our devops team)
3. set the GHCR_SECRET GH action secret in this repo to be a long-lived PAT for a different account (https://github.com/clockworklabs/SpacetimeDB/settings/secrets/actions). The current token will expire soon.
4. Make this check required, I think.

API and ABI breaking changes

None. CI-only changes.

Expected complexity level and risk

2

Testing

• The CI runs and seems to pass!