nix-shell: Set $IN_NIX_SHELL
nix-shell: Set $IN_NIX_SHELL

This allows scripts to distinguish between a real build and a Nix
shell.

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