fix for DS_ENV issue (#4992)
When there exist two `.deepspeed_env` files (e.g., at `~/.deepspeed_env`
and `./.deepspeed_env`), only the local version will be read and used.
The behavior changed with the support of `$DS_ENV`
(https://github.com/microsoft/DeepSpeed/pull/4006). This PR allows
multuple `.deepspeed_env` files from all the
`DEEPSPEED_ENVIRONMENT_PATHS` paths to be read and used when launching.
/cc @loadams @mrwyattii
