BuildOptions: store option tuples in a Set

As options are stored in an object owned by the eigenclass of a formula,
options defined in the Formula#options method can be added multiple
times if the formula is instantiated multiple times.

Store them in a set to prevent duplicates.

Fixes Homebrew/homebrew#14133.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/formula_support.rb

@@ -162,7 +162,7 @@ class BuildOptions
     @args = Array.new(args)
     # Extend it into an ARGV extension
     @args.extend(HomebrewArgvExtension)
-    @options = []
+    @options = Set.new
   end
 
   def add name, description=nil