Jenkins + Docker Improvements

[ravi-mosaicml]

1. Switch to using command2 to run tests, which supports the ephemeral storage k8s parameter and adds timestamp markings to build. Ephemeral storage should prevent tests from non-deterministically running out of disk space, since now K8S will ensure that the node has this much space available.
2. Add the build discarder config to the Jenkinsfile
3. Re-enable the conda builds. It's slow -- I know -- but it only runs whenever the dependencies (setup.py or meta.yaml) changes.
4. Using the merge target instead of head. Closes Switch to /merge instead of /head for forked PRs #268.
5. Printing the build URL for spawned jobs in the logs. Closes https://github.com/ravi-mosaicml/todo/issues/13.
6. Not installing node globally in the dockerfile.
7. If building the docker image, run the jobs that depend on the image only after the image is built, not before.
8. Attempted to fix bugs when merging into dev / main -- specifically guarded the use of the pullRequest variable so it doesn't crash when it's not defined. Also skipping (most) checks on the merge commit, since there is no need to re-run them.
9. Move the docker pyright matrix into composer.
10. Added milestones to automatically cancel old builds on the same PR.