nix-shell: Add --impure flag
nix-shell: Add --impure flag

This is currently the default, but I might change that to --pure in
the future.

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