build: assign region at runtime in e2e tests

[sobolk]

Description of changes

This PR's goal is to remove as much of CLI_REGION assignment from generated e2e workflow as possible to free up space (cb yml files have hard size limit).
Therefore:

1. Split e2e test script does not assign region unless test is force-pinned to some region.
2. Adds a script that assigns region at runtime using job identifier.

Issue #, if available

Description of how you validated changes

E2E test run

From handful of jobs:

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.

[sobolk]

finally