Use "$out/var/lib" as LOCALSTATEDIR configuration value
by default intsead of "/var/lib"
as a way toward top-level-directory independent runtime.
Add input argument externalLocalStateDir to optionally specify the
path to external LOCALSTATEDIR if not null.
Add NixOS module option
programs.singularity.enableExternalLocalStateDir (default to true)
to use "/var/lib" as LOCALSTATEDIR.