roachtest: set `COCKROACH_TESTING_FORCE_RELEASE_BRANCH` only when necessary

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