(amplify): Enable the possibility to configure buildImage

Currently in codebuild it's possible to configure in the section _buildSpec_ the object _environment_ and its property _buildImage_.

This property is ignored in an amplify hosting app.

### Use Case
Codebuild, by default, uses a build image version that seems to be old.
Newer versions has many new dependencies installed.
What I'd like to configure is something like this:

```python
new amplify.App(this, 'test', {
sourceCodeProvider: ...,
buildSpec: codebuild.BuildSpec.fromObject({
version: '1.0',
frontend: {
....
buildSpec: codebuild.BuildSpec.fromObject({
version: '1.0',
frontend: {
environment: {
buildImage: LinuxBuildImage.STANDARD_5_0,
privileged: true,
computeType: ComputeType.SMALL,
....
},
```

The `buildImage: LinuxBuildImage.STANDARD_5_0` has some dependecies missing in the current, default, old version, like _jq_ (that seems to be exploited by many users in codebuild).

### Proposed Solution

Please include buildSpec/environment/buildImage or a way to configure the image used by codebuild.

The issue points to Amplify hosting's buildSpec/environment handling and the CodeBuild buildImage setting; begin by locating that integration and its existing configuration coverage. Confirm the change by making buildImage configurable for an Amplify app and verifying that the selected image is passed through to the hosting build.

Written by the indexing model from the issue text.