x/build: add NetBSD 10_BETA builders #57997
Description
Context: NetBSD 10 is currently in beta state. There are no official releases yet, though there are daily builds from the branch.
https://go.dev/cl/463123 is the first change that has a different code path on NetBSD 10 than on earlier versions. Perhaps we should have some NetBSD 10 builders, at the very least for amd64.
Creating the image would be a simple change on top of the existing x/build/env/netbsd_amd64 etc. Maybe we could have a parameter in there that specifies whether to build a -9 or -10 image.
/cc @bcmills @cagedmantis @golang/netbsd