Fix segfault in builtin fetchurl
Fix segfault in builtin fetchurl

The stack allocated for the builder was way too small (32 KB). This is
sufficient for normal derivations, because they just do some setup and
then exec() the actual builder. But for the fetchurl builtin
derivation it's not enough. Also, allocating the stack on the caller's
stack was fishy business.

bennofs/nix,shlevy/nix,bennofs/nix,vizanto/nix,ttuegel/nix,ysangkok/nix,dtzWill/nix,ttuegel/nix,wmertens/nix,layus/nix,rycee/nix,pikajude/nix,NixOS/nix,vcunat/nix,peti/nix,ehmry/nix,layus/nix,NixOS/nix,ehmry/nix,wmertens/nix,peti/nix,dezgeg/nix,dtzWill/nix,layus/nix,dtzWill/nix,shlevy/nix,ehmry/nix,NixOS/nix,garbas/nix,NixOS/nix,dezgeg/nix,bennofs/nix,vizanto/nix,dtzWill/nix,zimbatm/nix,peti/nix,rycee/nix,vcunat/nix,shlevy/nix,ysangkok/nix,zimbatm/nix,layus/nix,zimbatm/nix,shlevy/nix,ysangkok/nix,ehmry/nix,peti/nix,ehmry/nix,garbas/nix,bennofs/nix,pikajude/nix,NixOS/nix,vizanto/nix,zimbatm/nix,dezgeg/nix,vcunat/nix,garbas/nix,ehmry/nix,dezgeg/nix,shlevy/nix,pikajude/nix,dtzWill/nix,garbas/nix,vizanto/nix,vcunat/nix,rycee/nix,ttuegel/nix,wmertens/nix,garbas/nix,wmertens/nix,rycee/nix,ttuegel/nix,dezgeg/nix,layus/nix,rycee/nix,peti/nix