[Nix] Factor out dev env variable logic to another file

Summary:
Move development environment variable logic to a separate file.

This is to move more shell logic into files which can be checked with CI tools
for shell code.

https://linear.app/comm/issue/ENG-1681

Test Plan:

# Ensure you're not in a nix shell
[[ -n "$IN_NIX_SHELL" ]] && exit

nix develop

echo "$MYSQL_UNIX_PORT"
# points to something like "$HOME/.local/share/MariaDB/mysql.sock

• Shellcheck CI

Reviewers: abosh, varun, O1 Nix, ashoat

Reviewed By: abosh, O1 Nix

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D4903