Ignore non-JavaExec run task when finding application's main class

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

spring-boot-integration-tests/spring-boot-gradle-tests/src/test/resources/main-class.gradle

@@ -14,10 +14,17 @@ apply plugin: 'spring-boot'
 group = 'installer'
 version = '0.0.0'
 
-bootRun {
-  main = 'org.springframework.boot.SpringApplication'
+if (project.hasProperty('bootRunMain')) {
+	bootRun {
+  		main = 'org.springframework.boot.SpringApplication'
+	}
 }
 
+if (project.hasProperty('nonJavaExecRun')) {
+	task run { }
+}
+
+
 jar {
 	baseName =  'installer'
 }