Add setting for java version used in process mode

[dst97]

Currently, when a child process is created, the child process is using the default java environemt (called by java). When tests require java agents, the use of the default java environment might not provide java agent support.

jagr/launcher/src/main/kotlin/org/sourcegrade/jagr/launcher/executor/ProcessWorker.kt

Line 57 in c47745b

.command("java", "-Dlog4j.configurationFile=log4j2-child.xml", "-jar", jagrLocation, "--child")

Provide an option in the configuration file to set the java version or, if possible, use the java environment the program was called with.