Merge pull request #132748 from NixOS/backport-127986-to-release-21.05

[Backport release-21.05] generate-expr-from-tarballs.pl: use nix-shell as script interpreter

pkgs/servers/x11/xorg/generate-expr-from-tarballs.pl

@@ -1,10 +1,7 @@
-#! /usr/bin/env perl
-
-# Usage:
-#
-# manually update tarballs.list
-# then run: cat tarballs.list | perl ./generate-expr-from-tarballs.pl
+#!/usr/bin/env nix-shell
+#!nix-shell --pure --keep NIX_PATH -i perl -p cacert nix perl
 
+# Usage: manually update tarballs.list then run: ./generate-expr-from-tarballs.pl tarballs.list
 
 use strict;
 use warnings;