refactor(executor): simplify handling of env vars

labgrid_config now accepts environment variables as env which can use location expansion for files provided under data:

labgrid_config(
    # ...
    env = {
        "LG_ENV": "$(rlocationpath :config.yaml)"
    },
    data = ["config.yaml"]
)

This allows us to handle env vars which are set by the executor in a single place. As a result, we stop doing it inside of a manager, which will unlock further refactoring to simplify the architecture (such as combining labgrid_config and labgrid_manager).

There are a few breaking changes, but they shouldn't affect many people as they're hidden behind the labgrid_config_toolchain macro.