t/perf: use configured PERL_PATH

t/perf: use configured PERL_PATH

Our benchmarks use a couple of Perl scripts to compute results. These
Perl scripts get executed directly, and as the shebang is hardcoded to
"/usr/bin/perl" this will fail on any system where the Perl interpreter
is located in a different path.

Our build infrastructure already lets users configure the location of
Perl, which ultimately gets written into the GIT-BUILD-OPTIONS file.
This file is being sourced by "test-lib.sh", and consequently we already
have the "PERL_PATH" variable available that contains its configured
location.

Use "PERL_PATH" to execute Perl scripts, which makes them work on more
esoteric systems like NixOS. Furthermore, adapt the shebang to use
env(1) to execute Perl so that users who have Perl in PATH, but in a
non-standard location can execute the script directly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>