Previously, the CI task invoked 2 mach commands and performed manual
gzip compressed. This was sub-optimal for a few reasons:

* Redundant moz.build evaluation made execution slower
* Potential for shell expansion to interfere with wildcards
* Made it harder to change exit code

By moving all logic into a mach command, we eliminate all the issues
around using shell.

Behavior of the new command and the CI task should be backwards
compatible.

MozReview-Commit-ID: HUFvLteKLOO

--HG--
extra : rebase_source : 85a49fc45eeaaac71dfb97fc22900a81ca9efa4b