threadscope: modernize formula and fix the build
threadscope: modernize formula and fix the build

- modernize the formula by using `install_cabal_package`
- avoid unnecessary reinstalls so that it doesn't attempt to install gtk
  without the `have-quartz-gtk` flag, leading to build failure

Closes #10.

Signed-off-by: ilovezfs <fbd54dbbcf9e596abad4ccdc4dfc17f80ebeaee2@icloud.com>

BrewTestBot/homebrew-gui