cmd/pack: address go vet reports

Use log.Fatalf for formatting directives instead of log.Fatal

Change-Id: Ia207b320f5795c63cdfa71f92c19ca6d05cc833f
Reviewed-on: https://go-review.googlesource.com/2160Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/pack/pack.go

@@ -183,7 +183,7 @@ func existingArchive(name string) bool {
 		if os.IsNotExist(err) {
 			return false
 		}
-		log.Fatal("cannot open file: %s", err)
+		log.Fatalf("cannot open file: %s", err)
 	}
 	checkHeader(fd)
 	fd.Close()
@@ -196,7 +196,7 @@ func checkHeader(fd *os.File) {
 	buf := make([]byte, len(arHeader))
 	_, err := io.ReadFull(fd, buf)
 	if err != nil || string(buf) != arHeader {
-		log.Fatal("%s is not an archive: bad header", fd.Name())
+		log.Fatalf("%s is not an archive: bad header", fd.Name())
 	}
 }