unix_chkpwd is designed to have a suid bit, but it's not possible to set it
for files in the store.  This patch tells unix_pam.so to look for
unix_chkpwd in setuid program directory on Guix System.

--- a/modules/module-meson.build
+++ b/modules/module-meson.build
@@ -114,7 +114,7 @@ if module == 'pam_unix'
   if enable_nis
     pam_module_src += ['yppasswd_xdr.c']
   endif
-  pam_module_c_args += ['-DCHKPWD_HELPER="@0@"'.format(sbindir / 'unix_chkpwd'),
+  pam_module_c_args += ['-DCHKPWD_HELPER="@0@"'.format('/run/privileged/bin' / 'unix_chkpwd'),
                         '-DUPDATE_HELPER="@0@"'.format(sbindir / 'unix_update')]
   pam_module_deps += [libcrypt, libselinux, libtirpc, libnsl]
 endif