tool/gocross: force use of our custom toolchain

The new 'toolchain' directive in go.mod can sometimes force
the use of an upstream toolchain against our wishes. Concurrently,
some of our dependencies have added the 'toolchain' directive, which
transitively adds it to our own go.mod. Force all uses of gocross to
ignore that directive and stick to our customized toolchain.

Updates #cleanup

Signed-off-by: David Anderson <danderson@tailscale.com>

tool/gocross/autoflags.go

@@ -191,6 +191,7 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 	env.Set("CC", cc)
 	env.Set("TS_LINK_FAIL_REFLECT", boolStr(failReflect))
 	env.Set("GOROOT", goroot)
+	env.Set("GOTOOLCHAIN", "local")
 
 	if subcommand == "env" {
 		return argv, env, nil