releng/pacman.conf: Enable parallel downloads

Now that pacman 6 is around, enable parallel downloads to speed up the installation
process.

configs/releng/pacman.conf

@@ -37,7 +37,7 @@ Architecture = auto
 # We cannot check disk space from within a chroot environment
 #CheckSpace
 #VerbosePkgLists
-#ParallelDownloads = 5
+ParallelDownloads = 5
 
 # By default, pacman accepts packages signed by keys that its local keyring
 # trusts (see pacman-key and its man page), as well as unsigned packages.