x/build: enable snapshotting for misc-compile builders

[mknyszek]

Right now misc-compile builders don't snapshot at all, forcing a re-run of make.bash. This is especially annoying for subrepos, since it adds a bunch of unnecessary build latency. We should at the very least enable snapshotting per builder name (which makes sense, since we'll have std built for the given GOOS/GOARCH).

[mknyszek]

CC @golang/release

[bcmills]

Since the Go compiler is a cross-compiler on every platform, we could alternately run all of the misc-compile subrepo builders with a common, already-snapshotted host toolchain (such as linux/amd64).

The x/build changes might be more invasive, but the end result would be more resource-efficient.