AppVeyor: Use the image "Visual Studio 2019" to use Ruby 3.0
AppVeyor: Use the image "Visual Studio 2019" to use Ruby 3.0

If `image` is omitted, an old one is used unexpectedly.

Signed-off-by: Takuro Ashie <71accf686b9a6e0fd369c5e62393a338d58863cd@clear-code.com>

fluent/serverengine