feat(ci): add xyz-build

[caugner]

Summary

(Extracted from #8366.)

Problem

Previously, we only had one test environment (stage), but this means we often have to wait for each other to "test on stage".

Solution

Add another environment xyz and a xyz-build workflow similar to stage-build.

Note: The xyz environment does not have a dedicated rumba, so it doesn't currently support logging in.

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How did you test this change?

Will run another xyz build now.

[LeoMcA]

My suggested change is my one real concern, everything else is nits/can be discussed and fixed later

[LeoMcA]

At some point it might be good to turn steps like this into shell scripts to: reduce duplication across workflows, and allow running things locally more easily