run macports check after failed build instead of before every build

Closes Homebrew/homebrew#28717.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/install.rb

@@ -93,7 +93,6 @@ module Homebrew extend self
     check_ppc
     check_writable_install_location
     check_xcode
-    check_macports
     check_cellar
   end
 
@@ -121,5 +120,6 @@ module Homebrew extend self
     opoo e.message
   rescue CannotInstallFormulaError => e
     ofail e.message
+    check_macports
   end
 end