This switches our gokrazy builds to use a new variant of cmd/gok called
opinionated about using monorepos: https://github.com/bradfitz/monogok
And with that, we can get rid of all the go.mod files and builddir forests
under gokrazy/**.
Updates #13038
Updates gokrazy/gokrazy#361
Change-Id: I9f18fbe59b8792286abc1e563d686ea9472c622d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
