change setInstance method to static

Signed-off-by: zane-neo <zaniu@amazon.com>

Author: zane-neo

distribution/tools/keystore-cli/src/test/java/org/opensearch/bootstrap/BootstrapTests.java

@@ -150,7 +150,7 @@ public void testInitExecutionOrder() throws Exception {
 
         LogConfigurator.registerErrorListener();
         Bootstrap testBootstrap = new Bootstrap(mockThread, mockNode);
-        testBootstrap.setInstance(testBootstrap);
+        Bootstrap.setInstance(testBootstrap);
 
         Bootstrap.startInstance(testBootstrap);
 

server/src/main/java/org/opensearch/bootstrap/Bootstrap.java

@@ -94,7 +94,7 @@ final class Bootstrap {
     private final Spawner spawner = new Spawner();
 
     // For testing purpose
-    void setInstance(Bootstrap bootstrap) {
+    static void setInstance(Bootstrap bootstrap) {
         INSTANCE = bootstrap;
     }