cmd/dist: do not leave go-tool-dist-* temporary directories behind

Change-Id: I3f6ba5591130b2c4762d33bd4553220765ad9fc5
Reviewed-on: https://go-review.googlesource.com/2996Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/dist/util.go

@@ -397,6 +397,7 @@ func main() {
 
 	xinit()
 	xmain()
+	xexit(0)
 }
 
 // xsamefile reports whether f1 and f2 are the same file (or dir)