fake-nss: Add support for extra passwd and group lines

pkgs/build-support/fake-nss/default.nix

···
 # Useful when packaging binaries that insist on using nss to look up
 # username/groups (like nginx).
 # /bin/sh is fine to not exist, and provided by another shim.
-{ symlinkJoin, writeTextDir, runCommand }:
+{ lib, symlinkJoin, writeTextDir, runCommand, extraPasswdLines ? [], extraGroupLines ? [] }:
 symlinkJoin {
   name = "fake-nss";
   paths = [
     (writeTextDir "etc/passwd" ''
       root:x:0:0:root user:/var/empty:/bin/sh
-      nobody:x:65534:65534:nobody:/var/empty:/bin/sh
+      ${lib.concatStrings (map (line: line + "\n") extraPasswdLines)}nobody:x:65534:65534:nobody:/var/empty:/bin/sh
     '')
     (writeTextDir "etc/group" ''
       root:x:0:
-      nobody:x:65534:
+      ${lib.concatStrings (map (line: line + "\n") extraGroupLines)}nobody:x:65534:
     '')
     (writeTextDir "etc/nsswitch.conf" ''
       hosts: files dns