[nix] Launch redis-server from REDIS_CACHE_DIR

Summary:
Redis will persist application files (eg dump.rdp) to disk from wherever it was launched. We're currently launching from the root of the repo, so I'm constantly getting a dump.rdp in my git working copy which is annoying (as would everyone else if they were actually using Redis/MariaDB via Nix...).

Instead we're cding to $REDIS_CACHE_DIR before launching Redis, so dump.rdp will live there and outside of the root of the repo.

Test Plan: dump.rdp appearing in $REDIS_CACHE_DIR instead of /comm after making this change + killing Redis:

Reviewers: jon, O1 Nix

Reviewed By: jon, O1 Nix

Subscribers: bartek, ashoat, tomek

Differential Revision: https://phab.comm.dev/D7016