my-env: Provide explicit store path for "rm".

If you do not provide any dependencies which don't propagate coreutils,
you'll get "rm: No such file or directory". Like for example by using
this:

nix-build -E '(import <nixpkgs> {}).myEnvFun {
    name = "mybrokentest";
    buildInputs = [];
}'

Thanks to "ewemoa" from the #nixos IRC channel for spotting this.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/misc/my-env/default.nix

@@ -131,7 +131,7 @@ mkDerivation {
         echo "\$tmp/script";
         source "\$tmp/script";
       fi
-      rm -fr "\$tmp"
+      ${pkgs.coreutils}/bin/rm -fr "\$tmp"
       ${extraCmds}
 
       nix_cleanup() {