update connection string

Author: lichuhub

swa-db-connections/staticwebapp.database.config.json

@@ -2,7 +2,7 @@
   "$schema": "https://github.com/Azure/data-api-builder/releases/download/v1.3.19/dab.draft.schema.json",
   "data-source": {
     "database-type": "cosmosdb_nosql",
-    "connection-string": "",
+    "connection-string": "@env('DATABASE_CONNECTION_STRING')",
     "options": {
       "database": "INT2",
       "container": "engagements",