more portable perl shebangs (#7701)

same motivation as in #5816:

- distributions have scripts to rewrite shebangs on installation and
  they know what locations to rely on.
- For tests/compilation we should rather rely on the user to have setup
  there PATH correctly.

test/udev-test.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 
 # udev test
 #

tools/check-includes.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl
+#!/usr/bin/env perl
 #
 # checkincludes: Find files included more than once in (other) files.
 # Copyright abandoned, 2000, Niels Kristian Bech Jensen <nkbj@image.dk>.