nix-env: Ignore manifest.nix when recursing into ~/.nix-defexpr
nix-env: Ignore manifest.nix when recursing into ~/.nix-defexpr

Channels are implemented using a profile now, and profiles contain a
manifest.nix file.  This should be ignored to prevent bogus packages
from showing up in nix-env.

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