rcm: Use Nix store path for RCM_LIB

The shell scripts in the rcm package use a relative path in order to
find the rcm.sh library.  That works fine when rcm is installed with
nix-env -- ~/.nix-profile/share/rcm exists as expected -- but does not
work when installed via /etc/nixos/configuration.nix.  In the
system-wide case, /run/current-system/sw/share does not contain the rcm
directory.