nixos/pam: mount encrypted home earlier

This patch was done by curro:

The generated /etc/pam.d/* service files invoke the pam_systemd.so
session module before pam_mount.so, if both are enabled (e.g. via
security.pam.services.foo.startSession and
security.pam.services.foo.pamMount respectively).

This doesn't work in the most common scenario where the user's home
directory is stored in a pam-mounted encrypted volume (because systemd
will fail to access the user's systemd configuration).