gradm: fix gradm_pam path

We alredy rewrote /sbin/gradm, which technically matches
/sbin/gradm_pam, so this ends up working exactly as we want. Otherwise
we rewrite twice and gradm can't execute the PAM module with '-p'

Signed-off-by: Austin Seipp <aseipp@pobox.com>
gradm: learn of all accesses for /nix/store

This is necessary for gradm's learning mode to work, as otherwise the
/nix/store directory is marked hidden, which causes the kernel to reject
the linker loading ld-linux.so

Signed-off-by: Austin Seipp <aseipp@pobox.com>