unrar: Fix for cross

Just read tool variables from environment,
don't try to set them at all.

pkgs/tools/archivers/unrar/default.nix

···
   };
 
   postPatch = ''
-    sed 's/^CXX=g++/#CXX/' -i makefile
+    substituteInPlace makefile \
+      --replace "CXX=" "#CXX=" \
+      --replace "STRIP=" "#STRIP=" \
+      --replace "AR=" "#AR="
   '';
 
   buildPhase = ''