shadow: add runtime-shell.patch

This gets the right shell in the closure.

By default, shadow wants to pull in the bash we are building it with,
while we would prefer it used the runtime bash.

pkgs/os-specific/linux/shadow/runtime-shell.patch

@@ -0,0 +1,13 @@
+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])