e2e tests on main should run flatcar tests

**What steps did you take and what happened:**
Triggering e2e tests on main only runs generic tests.

**What did you expect to happen:**
Flatcar tests should also run.

**Anything else you would like to add:**
We do not need to e2e test every commit in main (we already run e2e tests on all relevant PRs) but it would be nice to have a full e2e pass when making a release.

**Environment:**
e2e