doctor: use Formula.installed directly

Before
```
brew doctor  9.83s user 0.52s system 76% cpu 13.563 total
```

After
```
brew doctor  0.25s user 0.16s system 9% cpu 4.332 total
```

Closes Homebrew/homebrew#40770.

Signed-off-by: Xu Cheng <xucheng@me.com>