gnulib-tests/test-update-copyright.sh perl hardcoded path

[Gilles Espinasse]

path is hardcoded to /us/bin/perl in
YEAR=`/usr/bin/perl -e 'print [localtime]->[5] + 1900'`;

That fail on LFS style build as perl from toolchain is in /tools/bin and
final perl is build later

Gilles