After build fails once, cannot trigger a new build.

**Describe the bug**
I'm trying to setup continuous deployment from a private GitHub monorepo. Because I'm unable to edit build settings as described in issue #455, the deployment fails because amplify.yml cannot be found. Even though it's suggested in that issue to save, deploy, then edit build settings, it's impossible to start a new build. I've tried everything from the manual webhooks to pushing to the repo. The deployment screen acts as in #1343 and has blank bars where the progress bars should be. The only option that seems to work is manual deployments.

**To Reproduce**
Steps to reproduce the behavior:
1. Connect frontend repo to backend, without being able to edit build settings.
2. Watch build fail.
3. Edit build settings to add amplify push to backend, generating amplify.yml.
4. Try to trigger a new build.

**Expected behavior**
A new build should be triggered with the new build settings.

**Desktop (please complete the following information):**
macOS Firefox 85.0

App ARN arn:aws:amplify:us-west-2:462943309901:apps/dbr8ayau0vo27

Reproduce the flow using the issue's steps: connect the frontend repo, let the build fail, edit amplify.yml settings, and try a new build through the deployment screen or manual webhook. Compare the blank progress-bar state with the expected new-build behavior and verify that a subsequent build starts with the updated settings.

Written by the indexing model from the issue text.