cmd/dist: fix mkdeps

Mistake introduced just before submitting CL 67650 that somehow
did not break a final pre-submit all.bash on my laptop.

Not sure why all.bash passes locally when mkdeps.go doesn't build.
I guess the test only runs on builders?

Change-Id: I18fb91ada47605035345ba4b2f5e360a5c4b7f6e
Reviewed-on: https://go-review.googlesource.com/67850
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/mkdeps.go

@@ -155,6 +155,7 @@ func importsAndDepsOf(pkgs ...string) (map[string][]string, map[string][]string)
 		args := []string{"list", "-tags", "cmd_go_bootstrap", "-f", "{{range .Imports}}import {{$.ImportPath}} {{.}}\n{{end}}{{range .Deps}}dep {{$.ImportPath}} {{.}}\n{{end}}"}
 		args = append(args, pkgs...)
 		cmd := exec.Command("go", args...)
+		t := strings.Split(target, "/")
 		cmd.Env = append(os.Environ(), "GOOS="+t[0], "GOARCH="+t[1])
 		out, err := cmd.Output()
 		if err != nil {