roachtest: set COCKROACH_TESTING_FORCE_RELEASE_BRANCH only when necessary

[renatolabs]

In, #97595, we started setting COCKROACH_TESTING_FORCE_RELEASE_BRANCH in clusters created with roachprod. That is not ideal because:

• is not standard behavior that customers would encounter
• can be confusing, especially if you are trying to upgrade a cluster created with a version of roachprod that predated that change
• makes it harder to reuse disk images that were created before this change.

We should:

• not be setting any of COCKROACH_TESTING_FORCE_RELEASE_BRANCH or COCKROACH_UPGRADE_TO_DEV_VERSION by default on roachprod clusters
• only set COCKROACH_TESTING_FORCE_RELEASE_BRANCH in upgrade-related roachtests. Unfortunately this will involve some manual work to figure out which tests are going to need to change due to the ad-hoc nature of roachtest and the lack of a unified API.

Jira issue: CRDB-25713

[blathers-crl]

cc @cockroachdb/test-eng