cmd/go: fix -a

The one line that you can't test easily was broken.
This manifested as a failure of a pre-existing test
in test.bash but I didn't notice it (there are a few other
long-standing failures that need to be fixed).

TBR=r
CC=golang-codereviews
https://golang.org/cl/146340044

src/cmd/go/pkg.go

@@ -690,7 +690,7 @@ func computeStale(pkgs ...*Package) {
 // "go1.X[.Y]" for Go releases, and "devel +hash" at tip.
 // Determine whether we are in a released copy by
 // inspecting the version.
-var isGoRelease = !strings.HasPrefix(runtime.Version(), "go1")
+var isGoRelease = strings.HasPrefix(runtime.Version(), "go1")
 
 // isStale reports whether package p needs to be rebuilt.
 func isStale(p *Package, topRoot map[string]bool) bool {