pathname: fix rmdir_if_possible with mount points

Attempting to rmdir a directory that is a mount point results in EBUSY,
not EACCES or ENOENT, so also rescue EBUSY in rmdir_if_possible.

Library/Homebrew/extend/pathname.rb

@@ -255,7 +255,7 @@ class Pathname
     else
       false
     end
-  rescue Errno::EACCES, Errno::ENOENT
+  rescue Errno::EACCES, Errno::ENOENT, Errno::EBUSY
     false
   end