Previously, FindMainClassTask would look for a property named main
on any class named run. This was based on the assumption that the
run task would be a JavaExec task (typically provided by the
application plugin). If the run task was not a JavaExec task (more
accurately, if it did not have a main property) this would result in
a build failure due to trying to read a non-existent property.

This commit updates FindMainClassTask to only use the main property
of the run task if the task is a JavaExec task. This guarantees that
the property will exist on the task, and unlike using any property
named main on a task named run, also guarantee that its value will
refer to a Java class with a main method.

Closes gh-5501