Use source instead of release option for library compilation

Since https://bugs.openjdk.org/browse/CODETOOLS-7903775 added support for LIBRARY.properties, `--release=N` was included by default. That could lead to errors when system modules were part of the test scenario. Error messages emitted read like: `error: exporting a package from system module java.base is not allowed with --release`

Using `--source=N` will prevent that error to be reported.