Fixing problems in the previous commit. It didn't work.

pkgs/stdenv/generic/builder.sh

@@ -9,14 +9,16 @@ mkdir $out
 echo "$preHook" > $out/setup
 cat "$setup" >> $out/setup
 
+if [ "$withNixImpure" == 1 ]; then
+    # sed wants \&\& for a &&
+    niximpure='test -f /niximpure/impure.sh \&\& . /niximpure/impure.sh'
+fi
+
 sed -e "s^@initialPath@^$initialPath^g" \
     -e "s^@gcc@^$gcc^g" \
     -e "s^@shell@^$shell^g" \
+    -e "s^@niximpure@^$niximpure^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/linux/default.nix

@@ -94,6 +94,7 @@ rec {
       initialPath = [bootstrapTools] ++ extraPath;
       fetchurlBoot = fetchurl;
       inherit gcc;
+      withNixImpure = if platform ? nixImpure then platform.nixImpure else false;
       # Having the proper 'platform' in all the stdenvs allows getting proper
       # linuxHeaders for example.
       extraAttrs = extraAttrs // { inherit platform; };