Request: Allow to set environment variables for the build stage from the web console.

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**Tell us about your request**
What do you want us to build?

**Ability to pass environment variables for the build stage in App Runner directly from the web console**.

Use case:

I have 2 App Runner services, one for production and one for stage. Each one of these require specific environment variables for the application to build propertly. I can't use a single `apprunner.yml` configuration file because there is no way to specify the correct environment variables for each service. Having the ability to pass environment variables in the build stage per service will solve this problem.


**Describe alternatives you've considered**

Allow to set a path to the `apprunner.yml` file when creating the service as an alternative to it being automatically read by App Runner, that way we could specify where is the file in the repository for each service. For example: `apprunner.prod.yml` and `apprunner.stage.yml`.


**Additional context**
Anything else we should know?

This is a related issue: https://github.com/aws/apprunner-roadmap/issues/182

Thanks!.