fix command

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57062 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/make-snapshot

@@ -45,7 +45,7 @@ PACKAGES = {
 }
 
 if system("7z", out: IO::NULL)
-  PACKAGES["gzip"] = %w".tar.gz 7z a -tgzip -mx"
+  PACKAGES["gzip"] = %w".tar.gz 7z a dummy -tgzip -mx -so"
   PACKAGES["zip"]  = %w".zip    7z a -tzip  -mx"
 end
 if gzip = ENV.delete("GZIP")