brew-pin: Fix bug in `FormulaPin#pinned?`

Since `pinned?` was using `File#exists?`, broken symlinks left by force-upgraded
formulae made `pinned?` return false while `pin` returned with an error because
it could not overwrite the broken link. This is fixed by using `File#symlink?`
instead, which returns true for broken symlinks and false only when they do not
exist.

Closes Homebrew/homebrew#18858.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/formula_pin.rb

@@ -27,7 +27,7 @@ class FormulaPin
   end
 
   def pinned?
-    @path.exist?
+    @path.symlink?
   end
 
   def pinable?