shadow: use /bin/sh as default shell (#432117)

nixos/tests/sssd-ldap.nix

···
               objectClass: posixAccount
               userPassword: ${testPassword}
               homeDirectory: /home/${testUser}
+              loginShell: /run/current-system/sw/bin/bash
               uidNumber: 1234
               gidNumber: 1234
               cn: ""

pkgs/by-name/sh/shadow/package.nix

···
   lib,
   stdenv,
   fetchFromGitHub,
-  runtimeShell,
   nixosTests,
   autoreconfHook,
   bison,
···
     "man"
   ];
 
-  RUNTIME_SHELL = runtimeShell;
-
   nativeBuildInputs = [
     autoreconfHook
     bison
···
     ./keep-path.patch
     # Obtain XML resources from XML catalog (patch adapted from gtk-doc)
     ./respect-xml-catalog-files-var.patch
-    ./runtime-shell.patch
     ./fix-install-with-tcb.patch
   ];
 
-  # The nix daemon often forbids even creating set[ug]id files.
   postPatch = ''
+    # The nix daemon often forbids even creating set[ug]id files
     sed 's/^\(s[ug]idperms\) = [0-9]755/\1 = 0755/' -i src/Makefile.am
+
+    # The default shell is not defined at build time of the package. It is
+    # decided at build time of the NixOS configration. Thus, don't decide this
+    # here but just point to the location of the shell on the system.
+    substituteInPlace configure.ac --replace-fail '$SHELL' /bin/sh
   '';
 
   # `AC_FUNC_SETPGRP' is not cross-compilation capable.

pkgs/by-name/sh/shadow/runtime-shell.patch

···
-diff --git a/configure.ac b/configure.ac
-index e4c6aaec..03883ad7 100644
---- a/configure.ac
-+++ b/configure.ac
-@@ -682,7 +682,7 @@ if test "$enable_utmpx" = "yes"; then
- 	          [Define if utmpx should be used])
- fi
- 
--AC_DEFINE_UNQUOTED(SHELL, ["$SHELL"], [The default shell.])
-+AC_DEFINE_UNQUOTED(SHELL, ["$RUNTIME_SHELL"], [The runtime shell.])
- 
- AM_GNU_GETTEXT_VERSION(0.16)
- AM_GNU_GETTEXT([external], [need-ngettext])