Give more stack size to sbt in the AppVeyor build.
Give more stack size to sbt in the AppVeyor build.

In the process, update it a bit to use Scala 2.12.11 and the sbt
launcher version 1.3.12.

gzm0/scala-js,sjrd/scala-js,scala-js/scala-js,scala-js/scala-js,sjrd/scala-js,sjrd/scala-js,gzm0/scala-js,scala-js/scala-js