Change Github Action main.yaml to build with default Kotlin-core (1.5.32) and only run tests with other core

[cowtowncoder]

So: the initial matrix build will both build and test module against different kotlin-core versions.
But what we really want is to actually use same build (with specific kotlin-core) against different Kotlin versions.

This is currently necessary to test Kotlin 1.4 support but is in general needed to better verify compatibility.