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/pam_unix/Makefile.in
+++ b/modules/pam_unix/Makefile.in
@@ -651,1 +651,1 @@
-	-DCHKPWD_HELPER=\"$(sbindir)/unix_chkpwd\" \
+	-DCHKPWD_HELPER=\"/run/setuid-programs/unix_chkpwd\" \