More explicit handling of jps executable on Windows

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterFormationTasks.groovy

@@ -286,13 +286,15 @@ class ClusterFormationTasks {
             onlyIf { pidFile.exists() }
             // the pid file won't actually be read until execution time, since the read is wrapped within an inner closure of the GString
             ext.pid = "${ -> pidFile.getText('UTF-8').trim()}"
-            File jps = getJpsExecutableByName("jps")
-            if (!jps.exists()) {
+            File jps
+            if (Os.isFamily(Os.FAMILY_WINDOWS)) {
                 jps = getJpsExecutableByName("jps.exe")
+            } else {
+                jps = getJpsExecutableByName("jps")
+            }
             if (!jps.exists()) {
                 throw new GradleException("jps executable not found; ensure that you're running Gradle with the JDK rather than the JRE")
             }
-            }
             commandLine jps, '-l'
             standardOutput = new ByteArrayOutputStream()
             doLast {