We currently use an container image for `test-devices` job only; while
all other jobs use machine executor.

This allows us to switch golang and protoc verions easily without
manually managing Docker images (which requires building them manually
on a dev machines, etc).  All that while, we install dependencies on
every build in all other jobs..

`test-devices` now is one of the fastest jobs and isn't a constraint or
a bottleneck, so increasing its overhead by few seconds doesn't hurt the
overall developer iteration.

If we split tests effectively later, we can revisit.