Can't use environment variables in custom headers section of amplify.yml

Environment variables are not interpolated (i.e. their names are treated as literals) in the `customHeaders` section of amplify.yml (at least when `amplify.yml` is hosted in Amplify Console and not in the git repo).

For example, if `amplify.yml` contains:
```
env:
variables:
TRUSTED_DOMAIN: 'https://trusted.example.com'
frontend:
...
...
customHeaders:
headers:
- key: 'Access-Control-Allow-Origin'
value: ${TRUSTED_DOMAIN}
```
then when https://trusted.example.com tries to fetch a file from this app it will be blocked by CORS because the app is sending the header value "${TRUSTED_DOMAIN}" rather than "https://trusted.example.com":
```
Access to fetch at 'https://this.amplify.app.com/foobar.js' from origin 'https://trusted.example.com' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header contains the invalid value '${TRUSTED_DOMAIN}'. Have the server send the header with a valid value, or, if an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
```
The reason I want to use an environment variable in the `customHeaders` section is because I am setting up this Amplify Console app using Cloudformation, and I do not know the value of $TRUSTED_DOMAIN in advance of Cloudformation running - its value will be specific to a deployment environment, hence wanting to use it as an environment variable. The above example shows the environment variable being set in `amplify.yml` to demonstrate the point, but in my case the environment variable will be set by Cloudformation.

I can't find any documentation that states where in `amplify.yml` environment variables can be used. I assumed they can be used anywhere in the file but it looks like that's not the case. I notice that they can't be used in the `artifacts` section of `amplify.yml` either. This should either be fixed, or the documentation should make it clear where they can and can't be used and why.

Start by reproducing the behavior with an amplify.yml hosted in Amplify Console, focusing on env.variables, customHeaders, and artifacts. Determine whether environment-variable interpolation is supported in those sections; done means either the values are expanded correctly or the documentation clearly identifies the supported and unsupported locations.

Written by the indexing model from the issue text.