Allow platform properties in worker configs to be templated

[aaronmondal]

At the moment we need to do some fairly cloud-unfriendly gymnastics to dynamically set the value field in worker configs. For instance:

https://github.com/TraceMachina/nativelink/blob/00ff4a088365e616e6094c85d99d999a039338b8/deployment-examples/kubernetes/02_application.sh#L7-L15

This should instead be configurable via environment variables so that we can pass these values via yaml files directly.