Don't cache the final build/test step

[talex5]

If you re-build a successful run, you probably want to try the tests again (to see if they're flaky). We should probably remove the last line from the generated Dockerfile and run it using docker run instead, so there's no caching.

A side-effect of this is that we'd run the build step twice for PRs (once in the upstream repository and once in the fork). However, the final step is usually fairly quick, and it would mean that you get to see the test output in both places.