fix(agent): Fix build script cache

Fix agent JAR index generation gradle task to avoid breaking build cache.
The task generates an index into its input folder, making the tasks always outdated and mandatory to execute for each build.

Author: PerfectSlayer

dd-java-agent/build.gradle

@@ -162,12 +162,13 @@ shadowJar generalShadowJarConfig >> {
 }
 
 tasks.register('generateAgentJarIndex', JavaExec) {
+  def indexName = 'dd-java-agent.index'
   def contentDir = "${sourceSets.main.output.resourcesDir}"
-  def indexFile = "${contentDir}/dd-java-agent.index"
+  def indexFile = "${contentDir}/${indexName}"
 
   it.group = 'Build'
   it.description = "Generate dd-java-agent.index"
-  it.inputs.files(contentDir)
+  it.inputs.files(fileTree(contentDir).exclude(indexName))
   it.outputs.files(indexFile)
   it.mainClass = 'datadog.trace.bootstrap.AgentJarIndex$IndexGenerator'
   it.classpath = project.configurations.shadowInclude