postBuild script is running 30 seconds before actually finishing building

### Before opening, please confirm:

- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-console/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-console/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-console/blob/master/CONTRIBUTING.md).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.

### App Id

d3al4tzclcnvw3

### Region

us-west-2

### Amplify Console feature

Build settings

### Describe the bug

We are running a webhook on `postBuild` to invalidate a cloudfront cache, but notice it actually runs 30 seconds before the build time is actually finished (and assets are deployed). Therefor we can't properly time the invalidation.

Example. Today the invalidation webhook ran at 9:05:40 , but the build didnt actually finish until 9:06:10

Output from build (add this up and equals 9:06:10 - 30s after postBuild actually ran):
Started at 10/26/2021, 8:45:44 AM
Build duration 20 minutes 26 seconds

### Expected behavior

Run `postBuild` after the build is actually finished

### Reproduction steps

Rebuild master
Wait for `postBuild` step to run
Compare that to actual time it takes to finish and deploy assets

### Build Settings

```yaml
version: 1
frontend:
phases:
preBuild:
commands:
- npm install
build:
commands:
- npm run generate
postBuild:
commands:
- "curl -X POST -H 'Authorization: token ' --data '{\"event_type\": \"amplify_deploy\"}' https://api.github.com/repos///dispatches"
artifacts:
baseDirectory: dist
files:
- '**/*'
cache:
paths:
- node_modules/**/*
```

### Additional information

_No response_

Start by reproducing the timing with the provided app, region, rebuild steps, and build settings. Compare the postBuild webhook timestamp with the actual asset deployment completion, then verify that the webhook runs only after deployment has finished.

Written by the indexing model from the issue text.