Add runtime classpath as input to ThirdPartyAuditTask (elastic#110882)

The runtime classpath of `ThirdPartyAuditTask` was not being tracked as
an input which can cause issues with task output caching and build
avoidance.

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/precommit/ThirdPartyAuditTask.java

@@ -193,6 +193,11 @@ public Set<String> getMissingClassExcludes() {
     @SkipWhenEmpty
     public abstract ConfigurableFileCollection getJarsToScan();
 
+    @Classpath
+    public FileCollection getClasspath() {
+        return classpath;
+    }
+
     @TaskAction
     public void runThirdPartyAudit() throws IOException {
         Set<File> jars = getJarsToScan().getFiles();