Add embedding and whitespace jobs to buildkite (#41321)
* Add "Check whitespace" buildkite job
* Add `key` values for later `wait` blocks
* Add "embedding tests" to buildkite configuration
* Eliminate unnecessary work in buildkite builds
Don't bother to precompile the Julia system image like we normally would
want to if we're just going to run things once. Also use
`JULIA_NUM_CORES` instead of hard-coding `-j 6` into the buildsystem.
* Update embedding.yml
* Update whitespace.yml
