Repo fails to build without /m MSBuild flag
Looking at the MSBuild CLI doc, the m flag sets the number of concurrent processes to use. Without a number, it uses the number of processors on the PC. Without the flag itself, it'll be set to 1. It seems that our build will fail if ran completely synchronously.

It seems some kind of dependency (build ordering) isn't specified properly, as the VSIX container should exist after the CreateVsixContainer target executes. Investigating a binlog of this should be able to determine the problem.
I finally hit this in CI, even with the /m flag. So, it is definitely possible to hit randomly while the build runs on multiple processes.

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=6549829&view=results
Another failure here: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=6966047&view=results