Add Arthur Cohen to the list of people

[CohenArthur]

I simply ran cargo run add-people and fetched my zulip ID. Not sure if I need to do anything else, but happy to!

Needed for #1052

[vincenzopalazzo]

Ok CI is failing with

[WARN  rust_team::validate] couldn't perform checks relying on the Zulip API, some errors will not be detected
[WARN  rust_team::validate] cause: missing ZULIP_USER and/or ZULIP_TOKEN environment variables
[ERROR rust_team::validate] validation error: person `CohenArthur` is not a member of any team (active or archived), has no permissions, and is not an individual contributor to any repo
[ERROR rust_team] 1 validation errors found

Now I will cherry-pick your commit in my PR and see if we can solve the error :)

Thanks

[vincenzopalazzo]

Included in #1052

Thanks!

[rylev]

Closing this in favor of #1052