goinstall: fix -u for bzr

Changeset 7012:65601d7cac9e broke goinstall -u for bzr
branches.  This will fix it.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/3852041

src/cmd/goinstall/download.go

@@ -140,7 +140,7 @@ var svn = vcs{
 var bzr = vcs{
 	cmd:               "bzr",
 	metadir:           ".bzr",
-	checkout:          "checkout",
+	checkout:          "update",
 	clone:             "branch",
 	update:            "update",
 	updateReleaseFlag: "-rrelease",