fix: backend cli util to use the correct id

instead of using `pluginId: 'testPlugin'` now it uses the right id for
example `pluginId: 'test'`.

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

.changeset/shaggy-vans-return.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Fix the backend plugin to use correct plugin id

packages/cli/templates/default-backend-plugin/src/plugin.ts.hbs

@@ -10,7 +10,7 @@ import { createRouter } from './service/router';
  * @public
  */
 export const {{pluginVar}} = createBackendPlugin({
-  pluginId: '{{pluginVar}}',
+  pluginId: '{{id}}',
   register(env) {
     env.registerInit({
       deps: {