misc/ios: fix an error when getenv encounters unset variable

The error message should indicate the name of the unset variable,
rather than the value. The value will alwayse be empty.

Change-Id: I6f6c165074dfce857b6523703a890d205423cd28
Reviewed-on: https://go-review.googlesource.com/16555Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/ios/go_darwin_arm_exec.go

@@ -103,7 +103,7 @@ func main() {
 func getenv(envvar string) string {
 	s := os.Getenv(envvar)
 	if s == "" {
-		log.Fatalf("%s not set\nrun $GOROOT/misc/ios/detect.go to attempt to autodetect", s)
+		log.Fatalf("%s not set\nrun $GOROOT/misc/ios/detect.go to attempt to autodetect", envvar)
 	}
 	return s
 }