ICU-12811 Add CI workflow to retain caches that are flaky/costly to init

[echeran]

This PR is adding a workflow to ensure that certain CI caches are not evicted when they are flaky/costly to create.

The motivating reason is that PRs can have spurious build failures because the Maven build job cannot download all of the dependency artifacts due to flakiness. The job needs to be re-run in that case until all of the dependency artifacts can be successfully downloaded. This happens when the cache does not exist, which can happen because the cache is evicted automatically after 7 days of inactivity.

Note: Although ICU4J only has a few test dependencies, the Maven build uses plugins that are implemented with code that have many transitive dependencies.

By ensuring that the cache exists on the upstream default branch (main), the CI invocations for PRs will be able to reuse the cache from the default branch and load from it during the first run for that PR. This will help insulate contributors from job failures due to the flakiness.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-12811
• 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

[markusicu]

looks plausible...

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

~ Your Friendly Jira-GitHub PR Checker Bot

[markusicu]

lgtm tnx

[echeran]

fyi @srl295