chore: add env vars for Tiles local dev (#864)

## Problem
Missing environment variables from `.env-example` affecting users from
setting up local development environment.


## Solution
Update `.env-example`

Author: kevinkim-ogp

packages/backend/.env-example

@@ -37,6 +37,9 @@ POSTMAN_SMS_QPS_LIMIT_PER_CAMPAIGN=3
 # LOCAL DEV
 SERVE_WEB_APP_SEPARATELY=true
 CLOUDFLARE_TOKEN=...
+AWS_ACCESS_KEY_ID=DUMMYIDEXAMPLE
+AWS_SECRET_ACCESS_KEY=DUMMYEXAMPLEKEY
+AWS_REGION=some-aws-region
 S3_ENDPOINT=http://localhost:9000
 S3_COMMON_BUCKET=plumber-development-common-bucket
 S3_ACCESS_KEY=...