rmlint: Install to /bin, not /usr/bin

I discovered that after installing rmlint, I couldn't run it. This was because it was installed to /usr/bin (inside its Nix store path), and that was not in my path. After a quick search through /nix/store, I discovered that no other packages (at least of the ones I have installed) have a /usr/bin either, so this must not be good.

This commit fixes the problem by changing rmlint to install in /bin. I had to use preConfigure because rmlint's Makefile doesn't support PREFIX.