Add env var for platform of underlying device

[bluesliverx]

Description

Adds an environment variable for buildrunner.yaml files to be able to tell the underlying architecture of the system that buildrunner is running on (i.e. ARM, AMD, etc).

Related Issue

Motivation and Context

There is no way, currently, to tell the underlying architecture of the current machine that buildrunner is running on (i.e. ARM vs AMD). This adds an env var to be able to tell that, which can then be used in the buildrunner.yaml file to mark built artifacts as being for one platform vs another.

How Has This Been Tested?

I added a test step in the test files to ensure that the platform is being set to something and it errors if it is not set (tested locally without this change and with this change and it works properly).

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.