[CI] Fix GtkSharp 2.12 download

Author: sevoku

appveyor.yml

@@ -19,13 +19,13 @@ assembly_info:
   assembly_informational_version: '{version}'
 
 install:
-- if not exist gtk-sharp-2.12.42.msi appveyor DownloadFile https://dl.xamarin.com/GTKforWindows/Windows/gtk-sharp-2.12.42.msi
-- msiexec /i gtk-sharp-2.12.42.msi /qn /norestart
+- if not exist gtk-sharp-2.12.45.msi appveyor DownloadFile https://github.com/mono/gtk-sharp/releases/download/2.12.45/gtk-sharp-2.12.45.msi
+- msiexec /i gtk-sharp-2.12.45.msi /qn /norestart
 - if not exist gtk-sharp-2.99.3.msi appveyor DownloadFile https://download.gnome.org/binaries/win32/gtk-sharp/2.99/gtk-sharp-2.99.3.msi
 - msiexec /i gtk-sharp-2.99.3.msi /qn /norestart
 
 cache:
-- gtk-sharp-2.12.42.msi
+- gtk-sharp-2.12.45.msi
 - gtk-sharp-2.99.3.msi
 - packages