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\" \
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<div class='path'>path: <a href='/guix/log/?id=996deb819fb2207d4bcf07cafe3087dbbd24a9de'>root</a>/<a href='/guix/log/tests?id=996deb819fb2207d4bcf07cafe3087dbbd24a9de'>tests</a>/<a href='/guix/log/tests/guix-package-aliases.sh?id=996deb819fb2207d4bcf07cafe3087dbbd24a9de'>guix-package-aliases.sh</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/guix-package-aliases.sh?id=996deb819fb2207d4bcf07cafe3087dbbd24a9de&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-10-02 23:28:09 +0200'>2020-10-02</span></td><td><a href='/guix/commit/tests/guix-package-aliases.sh?id=ad54a73bb820a685f242976a86be63931789fa97'>guix build: Record package transformations in manifest entries.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-28 21:06:36 -0500'>2020-09-28</span></td><td><a href='/guix/commit/tests/guix-package-aliases.sh?id=d8934360d2453a403b5433e71d09188e4ed23b57'>tests: Simplify shell exit status negation;</a><span class='msg-avail'>...</span></td><td>Eric Bavier</td></tr>