Allow env update on task re-run

This is regarding the use case where a workflow execution failed because of environment related issues (i.e. endpoint unavailable, etc.). Endpoints and credentials can be passed on workflow invocation under the env param and then accessed by workflow tasks using the env() function. It is possible that the endpoint is replaced (i.e. different host/ip) as part of resolving the environment related issues or that the token passed as credential has expired. In these cases, the user will need to be able to update the env variables prior to re-run.