When running nix doctor on a healthy system, it just prints the store URI and
nothing else. This makes it unclear whether the system is in a good state and
what check(s) it actually ran, since some of the checks are optional depending
on the store type.

This commit updates nix doctor to print an colored log message for every check
that it does, and explicitly state whether that check was a PASS or FAIL to make
it clear to the user whether the system passed its checkup with the doctor.

Fixes #3084