Make sure local testing is still possible

With the new release, testing locally was broken because the
backend couldn't start. This is because we need to add a flag
to use the legacy backend.

Once the migration to the new backend system is done, this can
be removed

.changeset/nine-buckets-sneeze.md

@@ -0,0 +1,6 @@
+---
+'@spreadshirt/backstage-plugin-s3-viewer-backend': patch
+---
+
+Add legacy flag for local development. This flag can be removed once the
+migration to the new backend-system is done.

plugins/s3-viewer-backend/package.json

@@ -21,7 +21,7 @@
     "role": "backend-plugin"
   },
   "scripts": {
-    "start": "backstage-cli package start",
+    "start": "LEGACY_BACKEND_START=1 backstage-cli package start",
     "build": "backstage-cli package build",
     "lint": "backstage-cli package lint",
     "test": "backstage-cli package test",