[tools] Make run_perf.py non-executable

The script does not have the shebang and apparently running it without 'python'
on the command line produces some strange errors such as script trying to parse
itself as a suite config. Making it non-executable will make it clear that users
should prefix it with 'python' or 'vpython'.

R=machenbach@chromium.org

No-Try: true
No-Tree-Checks: true
Change-Id: I1d4373b8846034cf28b5fd02e60e9ed682330706
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605942Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61607}