treewide: change $IN_NIX_SHELL handling

... to be compatible with https://github.com/NixOS/nix/pull/933
while not breaking compatibility with current nix.

lib/trivial.nix

@@ -71,7 +71,7 @@ rec {
     + (if pathExists suffixFile then fileContents suffixFile else "pre-git");
 
   # Whether we're being called by nix-shell.
-  inNixShell = builtins.getEnv "IN_NIX_SHELL" == "1";
+  inNixShell = builtins.getEnv "IN_NIX_SHELL" != "";
 
   # Return minimum/maximum of two numbers.
   min = x: y: if x < y then x else y;

pkgs/development/tools/pypi2nix/default.nix

@@ -36,7 +36,7 @@ in stdenv.mkDerivation rec {
     # mv effect-*/effect                 $out/pkgs/effect
     mv requests-*/requests              $out/pkgs/
 
-    if [ "$IN_NIX_SHELL" != "1" ]; then
+    if [ -z "$IN_NIX_SHELL" ]; then
       if [ -e git-export ]; then
         mv git-export/src/pypi2nix      $out/pkgs/pypi2nix
       else