[Nix] Move daemon creation to separate script

Summary:
There's a desire to not have bash embedded
in a nix string, as it circumvents linting and some
CI checks. This is to pull the daemon creation
out to it's own separate file.

Test Plan:

# If you have mariadb setup through homebrew, then run:
brew services stop mariadb

nix develop

Should see:

$ nix develop
View MariaDB Logs: tail -f $HOME/.local/share/MariaDB/logs
Kill MariaDB server: pkill mariadbd
Welcome to Comm dev environment! :)
[15:03:57] jon@Jons-MacBook-Pro ~/comm/comm (jonringer/web-services)

Reviewers: O1 Nix, abosh, varun

Reviewed By: O1 Nix, abosh

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D4879