Fix third-party audit task for Gradle 3.4 (#23612)

In Gradle 3.4, the buildSrc plugin seems to be packaged into a jar before it is accessed by the rest of the build and the signatures file for the third-party audit task cannot be accessed as
getClass().getResource('/forbidden/third-party-audit.txt') then points to a file entry in a JAR, which cannot be loaded directly as a File object. This commit changes the third-party audit task to pass the content of the signatures file as a String instead.

buildSrc/src/main/groovy/org/elasticsearch/gradle/precommit/ThirdPartyAuditTask.groovy

@@ -209,9 +209,11 @@ public class ThirdPartyAuditTask extends AntTask {
         try {
             ant.thirdPartyAudit(failOnUnsupportedJava: false,
                             failOnMissingClasses: false,
-                            signaturesFile: new File(getClass().getResource('/forbidden/third-party-audit.txt').toURI()),
                             classpath: classpath.asPath) {
                 fileset(dir: tmpDir)
+                signatures {
+                    string(value: getClass().getResourceAsStream('/forbidden/third-party-audit.txt').getText('UTF-8'))
+                }
             }
         } catch (BuildException ignore) {}