streamline GraalVM version logic

[johanvos]

Substrate should decide what GraalVM version it needs to perform compilation, dependent on the projectConfiguration. The logic needs to be documented clearly, and can be the following:

• if the system env variable GRAALVM_HOME is defined, Substrate will first check if that version matches the requirements (e.g. on iOS, we need 20-dev). ** if the version is ok, Substrate will use that one ** if the version is not ok, go to next step (or bail?)
• if GRAALVM_HOME is not set, Substrate will decide what version to use, based on the projectConfiguration ** Substrate will then search its cache (if it finds a cache, which is not the case when running tests) for that version. If it finds that version, use it ** If no such version is found (or there is no cache), Substrate will download the required version (e.g. 19.3 from graalvm.org 20.0-dev from gluonhq)

[jperedadnr]

We have first bullet in place. Second one leads to a similar discussion as in #34 I believe this can be closed @johanvos ?