[Runtime Env] Make every ${ENV_VAR} replaceable

[rkooo567]

Why are these changes needed?

In the master, we only allow a few env vars specified via ${} translatable. This is unnecessarily too restrictive. This PR allows every env vars to be translated if they are specified via ${ENV_VAR}

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[rkooo567]

Looks like the Python implementation leaves the env var unchanged if it doesn't exist. E.g.,

unset ENV_VAR

# previous
${ENV_VAR} -> ""

# now
${ENV_VAR} -> ${ENV_VAR}

I feel like this should be fine (especially given this python implementation will work cross platform correctly). But if we have to keep this behavior, I can somehow write the custom implementation

[jjyao]

nice, using python's expandvars is fine

[jjyao]

Can you also update the doc (https://docs.ray.io/en/latest/ray-core/handling-dependencies.html) as well?

Currently it has

env_vars (Dict[str, str]): Environment variables to set. Environment variables already set on the cluster will still be visible to the Ray workers; so there is no need to include os.environ or similar in the env_vars field. By default, these environment variables override the same name environment variables on the cluster. You can also reference existing environment variables using ${ENV_VAR} to achieve the appending behavior. Only PATH, LD_LIBRARY_PATH, DYLD_LIBRARY_PATH, and LD_PRELOAD are supported. See below for an example:

Example: {"OMP_NUM_THREADS": "32", "TF_WARNINGS": "none"}

Example: {"LD_LIBRARY_PATH": "${LD_LIBRARY_PATH}:/home/admin/my_lib"}

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[rkooo567]

cc @jjyao it is ready for review.

[jjyao]

Do we have tests already?

[rkooo567]

@jjyao yes!

[rkooo567]

Merged it because the previous run succeeds, and I only made doc changes.