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>