gokrazy: use monorepo for gokrazy appliance builds (monogok)

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>

tstest/integration/nat/nat_test.go

@@ -81,7 +81,7 @@ func newNatTest(tb testing.TB) *natTest {
 		}
 	}
 
-	nt.kernel, err = findKernelPath(filepath.Join(modRoot, "gokrazy/natlabapp/builddir/github.com/tailscale/gokrazy-kernel/go.mod"))
+	nt.kernel, err = findKernelPath(filepath.Join(modRoot, "go.mod"))
 	if err != nil {
 		tb.Skipf("skipping test; kernel not found: %v", err)
 	}