Automatically test new builds with dataplane

[qmonnet]

Currently, we must manually test the images built in PRs with the dataplane repo, or merge and hope for the best (and revert if dataplane's CI detects issues with a new compile-env image).

Instead, it would be great to add a job to the build.yml workflow, after the new image has been built, to try that image by checking out the dataplane repo and building the dataplane, using the sterile environment provided by the new image.

[daniel-noland]

Fully agree. Will think about the best way to do this.