This helps users to debug whether their current profile symlinks are
correctly set.
Expected outputs look like this:
```
❯ sudo ./outputs/out/bin/nix doctor
[snip]
[PASS] All profiles are gcroots.
[PASS] Client protocol matches store protocol.
[INFO] You are trusted by store uri: local
[FAIL] Error: current generation cannot be discovered for profile:
'/nix/var/nix/profiles/default'
```
```
❯ ./outputs/out/bin/nix doctor
[snip]
[PASS] All profiles are gcroots.
[PASS] Client protocol matches store protocol.
[INFO] You are trusted by store uri: daemon
[PASS] You have 28 generations for profile
'/nix/var/nix/profiles/per-user/raito/profile'
The current generation number is '290'
```
Change-Id: I50c69cbeac3291d668f4c2332803411579adc944
Signed-off-by: Raito Bezarius <raito@lix.systems>
Co-authored-by: Qyriad <qyriad@qyriad.me>
