In a few places the tag-set specification for jobs could match
either amd64 or arm64 runners.  That non-determinism is bad,
especially when passing platform-specific artifacts around.

This is just a stop-gap measure.  Later we'll need to rethink our CI
in terms of the two platforms.

I didn't touch tag-sets with `condor`, as that will probably always be
just a single machine (which coordinates scheduling on others).