x/build: do the buildongce -make_basepin work automatically in the coordinator in the background #21305
Description
Whenever somebody creates a new GCE image, it's important for latency reasons to create a base disk from that VM image in the region we use to run buildlet VMs.
If such a disk exists, future disk creations are Copy-On-Write and get created nearly instantly.
If a disk doesn't (and there's no hidden cache disk in the region from a recent creation), then you wait a bit for the copy from storage.
So buildongce -make-basepin auto-creates our "basepin" disks if needed.
I don't know why I shoved that functionality into the buildongce tool. Maybe it should be its own tool.
But in any case, we should have references to it from each env/**/{README,Makefile} so people updating these images in the future know to run it, or have the Makefiles auto-run it.