`run_binary` doesn't seem to inherit environment variables from `--host_action_env`

I would expect `--host_action_env` variables to apply to `run_binary`.

Start by reproducing the interaction between `run_binary` and `--host_action_env`, checking whether the environment variables are visible when `run_binary` executes. Done means the reported variables are inherited as expected, with a regression test covering the behavior.

Written by the indexing model from the issue text.