[Nix] Reduce services noise when doing nix develop

Summary:
Avoid printing how to view logs and kill the service
when the services are already available.

In a similar spirit, only emit that it's waiting
for mariadb if it's actually waiting for mariadb.

This cuts down on every invocation of nix develop cluttering
the development workflow.

Depends on D4880

Test Plan:

pkill mariadb
pkill redis

nix develop
# Should print everything

exit # leave nix shell

nix develop
# should print only print welcome message

Reviewers: abosh, varun, O1 Nix, ashoat

Reviewed By: abosh, O1 Nix, ashoat

Subscribers: ashoat, tomek, atul, karol

Differential Revision: https://phab.comm.dev/D4881