add dependency on pacman-contrib

[nwmahoney]

this line in archive.sh calls pacsort, which is included in the pacman-contrib package (which was split out of the pacman package a while ago). Without pacsort, we see the following error in the logs:

==> Updating package index
/usr/bin/archive: line 207: pacsort: command not found
sort: write failed: 'standard output': Broken pipe
sort: write error