Allow enabling CUDF_SPILL via an environment variable

[ayushdg]

Prior to #1362 cudf spilling could be enabled either via client.run(cudf.set_option) or via the CUDF_SPILL env variable. #1362 adds a flag to enable cudf spilling that makes the process a lot simpler but doesn't work in cases where users are trying to enable spilling via the CUDF_SPILL environment variable rather than using the flag.
This results in the workers being initialized without cudf spill by default ignoring the env variable and leading to errors downstream.