Hi guys, I'm trying to get NixPkgs working on a RHEL7 box with sssd. /etc/nsswitch.conf is configured to lookup with the sssd module, so when a coreutils program like id or whoami runs it needs to have libnss_sss.so.2.

If I install sssd via nixpkgs, and then do:
```
LD_PRELOAD=/nix/store/<hash>sssd-1.16.0/lib/libnss_sss.so.2 whoami
```
everything works as expected.  What's the best way for me to get this into my stdenv so that everything depends on sssd and has it available on its rpath?