Temporarily disable Windows tests (flutter#3862)

Windows bots are currently broken for building Flutter desktop apps; see flutter/flutter#82032 and related issues.

This temporarily disables the tests; most PRs do not affect Windows, so we don't need the whole tree to continue to be closed while that's resolved.

.ci/dev/try_builders.json

@@ -3,7 +3,7 @@
       {
          "name":"Windows Plugins",
          "repo":"plugins",
-         "enabled":true
+         "enabled":false
       }
    ]
 }