chore: simplify manual pr script

[sobolk]

Description of changes

I keep running into weird corner cases with manual PR build like:

• if submitted from branch from main repo, branch ref is passed to CB
• if submitted from fork, current inference works well
• if submitted from fork but also pushed to e2e branch before, somehow ref to e2e branch lands in CB

In order to simplify this, and because we have control over submitting manual build. I define an env var that I pass from manual script that is then used in CB to infer that we're in PR workflow.

Issue #, if available

Description of how you validated changes

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.