Different runners have different JDKs installed, so using a hard-coded
list for
`toolchains.xml` doesn't work. With this change, the file is generated
based on the available `JAVA_HOME_*` environment variables.
Fixes#89
Thanks @hfhbd for the contribution!
Co-authored-by: hfhbd <22521688+hfhbd@users.noreply.github.com>