goinstall: use https for Google Code checkouts

This way, if you later want to check things in,
you can (with appropriate authorization).
Using plain http leads to the cryptic error
abort: HTTP Method Not Allowed.

R=r
CC=golang-dev
https://golang.org/cl/1983050

src/cmd/goinstall/download.go

@@ -58,7 +58,7 @@ func download(pkg string) (string, os.Error) {
 			// regexp only allows hg, svn to get through
 			panic("missing case in download: " + pkg)
 		}
-		if err := vcsCheckout(v, root+m[1], "http://"+m[1], m[1]); err != nil {
+		if err := vcsCheckout(v, root+m[1], "https://"+m[1], m[1]); err != nil {
 			return "", err
 		}
 		return root + pkg, nil