x/build: make OpenBSD use a memory filesystem for its workdir

[bradfitz]

The OpenBSD builders are some of the slowest.

See if it's faster with a memory filesystem for its workdir.

See #16951 (comment) for details. Basically just:

$ mount -t mfs -o rw,nodev,nosuid,-s=1G /dev/sd0b /tmp

Make the buildlet do that on start, like it already tweaks other system stuff.

/cc @mdempsky @4a6f656c

[bradfitz]

I tried this on an OpenBSD 6.4 amd64 instance and it works, but it's still not any faster. Continuing debugging in #29223