nixos.tests.pam-file-contents: fix build failure

This fixes the test for pam file contents.
The pam file started using fully-qualified paths in the following
commit.
7306423158ba8d6e5d5674c4d6a7fcfea3c75f84

nixos/tests/pam/pam-file-contents.nix

···
 
     testScript =
       builtins.replaceStrings
-        [ "@@pam_ccreds@@" "@@pam_krb5@@" ]
-        [ pkgs.pam_ccreds.outPath pkgs.pam_krb5.outPath ]
+        [ "@@pam@@" "@@pam_ccreds@@" "@@pam_krb5@@" ]
+        [ pkgs.pam.outPath pkgs.pam_ccreds.outPath pkgs.pam_krb5.outPath ]
         (builtins.readFile ./test_chfn.py);
   }
 )

nixos/tests/pam/test_chfn.py

···
 expected_lines = {
-    "account required pam_unix.so",
+    "account required @@pam@@/lib/security/pam_unix.so",
     "account sufficient @@pam_krb5@@/lib/security/pam_krb5.so",
     "auth [default=die success=done] @@pam_ccreds@@/lib/security/pam_ccreds.so action=validate use_first_pass",
     "auth [default=ignore success=1 service_err=reset] @@pam_krb5@@/lib/security/pam_krb5.so use_first_pass",
-    "auth required pam_deny.so",
+    "auth required @@pam@@/lib/security/pam_deny.so",
     "auth sufficient @@pam_ccreds@@/lib/security/pam_ccreds.so action=store use_first_pass",
-    "auth sufficient pam_rootok.so",
-    "auth sufficient pam_unix.so likeauth try_first_pass",
+    "auth sufficient @@pam@@/lib/security/pam_rootok.so",
+    "auth sufficient @@pam@@/lib/security/pam_unix.so likeauth try_first_pass",
     "password sufficient @@pam_krb5@@/lib/security/pam_krb5.so use_first_pass",
-    "password sufficient pam_unix.so nullok yescrypt",
+    "password sufficient @@pam@@/lib/security/pam_unix.so nullok yescrypt",
     "session optional @@pam_krb5@@/lib/security/pam_krb5.so",
-    "session required pam_env.so conffile=/etc/pam/environment readenv=0",
-    "session required pam_unix.so",
+    "session required @@pam@@/lib/security/pam_env.so conffile=/etc/pam/environment readenv=0",
+    "session required @@pam@@/lib/security/pam_unix.so",
 }
 actual_lines = set(machine.succeed("cat /etc/pam.d/chfn").splitlines())