Make prettier happy when generating plugins

I've noticed that creating a plugin and immediately running `prettier check` causes failures due to the missing semicolon in this file.

Signed-off-by: Julio Zynger <julio.zynger@soundcloud.com>

.changeset/olive-mayflies-scream.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Add semicolon in template to make prettier happy

packages/cli/templates/default-plugin/src/setupTests.ts

@@ -1,2 +1,2 @@
 import '@testing-library/jest-dom';
-import 'cross-fetch/polyfill'
+import 'cross-fetch/polyfill';