cmd/go: restore "go list" exit code for missing binary packages

Prior to CL 108156, "go list" exited with code 0 if there were missing binary packages.
Restore that behavior to fix the failing TestBinaryOnlyPackages.

Fixes golang/go#25784.

Change-Id: Iaeca6bcc840f5ac8f2a016cef131f5c59a8261a8
Reviewed-on: https://go-review.googlesource.com/117036Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -4217,7 +4217,6 @@ func TestFatalInBenchmarkCauseNonZeroExitStatus(t *testing.T) {
 }
 
 func TestBinaryOnlyPackages(t *testing.T) {
-	t.Skip("known failing test; see golang.org/issue/25784")
 	tooSlow(t)
 
 	tg := testgo(t)

src/cmd/go/internal/work/exec.go

@@ -405,6 +405,9 @@ func (b *Builder) build(a *Action) (err error) {
 		}
 		a.Package.Stale = true
 		a.Package.StaleReason = "missing or invalid binary-only package"
+		if b.IsCmdList {
+			return nil
+		}
 		return fmt.Errorf("missing or invalid binary-only package")
 	}