tests: Fix wrong channel name in nix-channel.sh.
tests: Fix wrong channel name in nix-channel.sh.

The $channelName variable passed to the channel builder is the last
portion of the URL and while that works in the previous test for
channels prior to #519, it doesn't work if the last portion is
nixexprs.tar.bz2.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

dtzWill/nix,shlevy/nix,joelmo/nix,wmertens/nix,ysangkok/nix,pikajude/nix,layus/nix,acowley/nix,zimbatm/nix,pikajude/nix,aucampia/nix,bjornfor/nix,wmertens/nix,layus/nix,puffnfresh/nix,garbas/nix,rowhit/nix,shlevy/nix,tchajed/nix,tchajed/nix,dtzWill/nix,tchajed/nix,dezgeg/nix,gridaphobe/nix,puffnfresh/nix,garbas/nix,zimbatm/nix,NixOS/nix,gebner/nix,ehmry/nix,layus/nix,bjornfor/nix,joelmo/nix,gridaphobe/nix,joelmo/nix,vizanto/nix,ysangkok/nix,NixOS/nix,shlevy/nix,ehmry/nix,peti/nix,bennofs/nix,offlinehacker/nix,ttuegel/nix,vcunat/nix,rowhit/nix,rycee/nix,wmertens/nix,dezgeg/nix,zimbatm/nix,NixOS/nix,vcunat/nix,vizanto/nix,bennofs/nix,dtzWill/nix,acowley/nix,wmertens/nix,offlinehacker/nix,joelmo/nix,dtzWill/nix,aucampia/nix,ysangkok/nix,rycee/nix,aucampia/nix,layus/nix,dtzWill/nix,acowley/nix,aucampia/nix,rycee/nix,garbas/nix,gridaphobe/nix,acowley/nix,offlinehacker/nix,bennofs/nix,bjornfor/nix,shlevy/nix,garbas/nix,puffnfresh/nix,rycee/nix,shlevy/nix,peti/nix,peti/nix,bennofs/nix,gridaphobe/nix,vizanto/nix,puffnfresh/nix,gebner/nix,zimbatm/nix,vcunat/nix,ehmry/nix,dezgeg/nix,layus/nix,ttuegel/nix,rowhit/nix,ehmry/nix,NixOS/nix,vizanto/nix,tchajed/nix,bjornfor/nix,peti/nix,NixOS/nix,dezgeg/nix,pikajude/nix,gebner/nix,ttuegel/nix,ttuegel/nix,ehmry/nix,peti/nix,ehmry/nix,dezgeg/nix,garbas/nix,gebner/nix,vcunat/nix,rowhit/nix,offlinehacker/nix,rycee/nix