nix doctor: reimplement profile warning without gcroot check
nix doctor: reimplement profile warning without gcroot check

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.

dtzWill/nix,dtzWill/nix,dtzWill/nix,dtzWill/nix,dtzWill/nix