Inspired by the homebrew command, shows a combination of debugging
information and warnings with potential issues with a nix installation.

A protocol mismatch can sometimes cause problems when using specific
features with an older daemon. For example:

Nix 2.0 changed the way files are compied to the store.  The daemon is
backwards compatible and can still handle older clients, however a 1.11
nix-daemon isn't forwards compatible.

The serve protocol used by LegacySSHStore has a different major and
shouldn't be compared to PROTOCOL_VERSION.

It's pretty easy to unintentionally install a second version of nix
into the user profile when using a daemon install.  In this case it
looks like nix was upgraded while the nix-daemon is probably still
unning an older version.

In most cases profiles that are in PATH should have a gcroot.

Not all store types LegacySSHStore support these operations and it
doesn't really make sense to check those.

Calculating roots seems significantly slower on darwin compared to
linux.  Checking for /profile/ links could show some false positives but
should still catch most issues.