feat(tap): Only tap homebrew/core if tap is empty

In order to make the operation faster, and add `--force` option to run correctly.

main.rb

@@ -74,11 +74,12 @@ def read_brew(*args)
   git 'config', '--global', 'user.name', user_name
   git 'config', '--global', 'user.email', user_email
 
-  # Always tap homebrew/core, otherwise brew can't find formulae
-  brew 'tap', 'homebrew/core'
-
-  # Tap the requested tap if applicable
-  brew 'tap', tap unless tap.blank?
+  if tap.blank?
+    brew 'tap', 'homebrew/core', '--force'
+  else
+    # Tap the requested tap if applicable
+    brew 'tap', tap
+  end
 
   # Append additional PR message
   message = if message.blank?