nix-shell: Execute shellHook if it exists

Since normal builds don't execute shellHook, this allows nix-shell
specific customisation.  Suggested by Domen.

scripts/nix-build.in

@@ -207,6 +207,7 @@ foreach my $expr (@exprs) {
             ($pure ? '' : 'p=$PATH; ' ) .
             'dontAddDisableDepTrack=1; ' .
             '[ -e $stdenv/setup ] && source $stdenv/setup; ' .
+            'if [ "$(type -t runHook)" = function ]; then runHook shellHook; fi; ' .
             ($pure ? '' : 'PATH=$PATH:$p; unset p; ') .
             'set +e; ' .
             '[ -n "$PS1" ] && PS1="\n\[\033[1;32m\][nix-shell:\w]$\[\033[0m\] "; ' .