build/windows: Do not allow cancelling itReinstall nor itUpdate

As said on cbe2f6be, Inno does not support modifying installations at
all, not even canceling/undoing modifications like reinstall and update.

This limitation become more apparent after the fix 24e42dd1 so
let's prevent the users being damaged by such major Inno pitfall.

build/windows/installer/gimp-setup.iss

@@ -2752,6 +2752,12 @@ begin
   case pCurStep of
     ssInstall:
     begin
+      //As on usUninstall, do not allow to cancel on itReinstall or itUpdate
+      //since Inno does not undo [InstallDelete] and RemoveDebugFiles()
+      if (InstallType = 'itReinstall') or (InstallType = 'itUpdate') then begin
+        WizardForm.CancelButton.Enabled := False;
+      end;
+
       if IsAdminInstallMode() then begin
         RestorePoint();
       end;