ICU-22722 Prevent running PR check enforcement after merging PRs

[echeran]

Post-merge CI workflows run on every commit after it merges to main. However, we also use the setting that interrupts a workflow run when a new commit gets added to the branch.

In order to make commits on main in the Github UI not look like failures, we should not run the enforce-all-checks job after merging to main. This is the right change to make because the sole purpose of the enforce-all-checks job is to have intelligent automated enforcement of CI jobs before merging (that is: as a requirement before a PR is allowed to merge).

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22722
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[FrankYFTang]

LGTM