Revisit bundler usage in E2E tests

Currently, App Engine, Cloud Run, and (I think) EventArc tests are classifed as E2E because they require access to gcloud. However, these tests aren't one-size-fits-all, and there are some issues. In particular, the test scripts all run bundler in the sample directory. This doesn't apply to some of the samples. For example, in the Cloud Run samples, the Gemfile is intended to be for the container being built and deployed to Cloud Run, not for the testing environment. As a result, for example, we are currently not able to install a newer version of Ruby (or bundler) in the deployment image because it has to match the versions used by the test harness. The test scripts should not assume that they must always run bundler in the sample directory.

The goal should be to allow bundler and Ruby to be updated in those samples (See #748)