2nd approach at getting a check for /niximpure in stdenv-updates

pkgs/stdenv/generic/builder.sh

@@ -13,6 +13,10 @@ sed -e "s^@initialPath@^$initialPath^g" \
     -e "s^@gcc@^$gcc^g" \
     -e "s^@shell@^$shell^g" \
     < $out/setup > $out/setup.tmp
+if [ -n "$withNixImpure" ]; then
+    sed -i -e 's^@niximpure@^test -f /niximupure/impure.sh && . /niximpure/impure.sh^g' \
+      $out/setup.tmp
+fi
 mv $out/setup.tmp $out/setup
 
 # Allow the user to install stdenv using nix-env and get the packages

pkgs/stdenv/generic/default.nix

@@ -1,5 +1,6 @@
 { system, name ? "stdenv", preHook ? "", initialPath, gcc, shell
 , extraAttrs ? {}, overrides ? (pkgs: {})
+, withNixImpure ? false
 
 , # The `fetchurl' to use for downloading curl and its dependencies
   # (see all-packages.nix).
@@ -26,7 +27,7 @@ let
 
         setup = setupScript;
 
-        inherit preHook initialPath gcc shell;
+        inherit preHook initialPath gcc shell withNixImpure;
 
         propagatedUserEnvPkgs = [gcc] ++
           lib.filter lib.isDerivation initialPath;

pkgs/stdenv/generic/setup.sh

@@ -270,6 +270,7 @@ elif [ "$NIX_BUILD_CORES" -le 0 ]; then
 fi
 export NIX_BUILD_CORES
 
+@niximpure@
 
 ######################################################################
 # Misc. helper functions.

pkgs/stdenv/linux/default.nix

@@ -282,6 +282,8 @@ rec {
     
     fetchurlBoot = fetchurl;
 
+    withNixImpure = if platform ? nixImpure then platform.nixImpure else false;
+    
     extraAttrs = {
       inherit (stdenvLinuxBoot3Pkgs) glibc;
       inherit platform;