rofl build sgx: Automatically define env variable with the latest state root

[matevz]

Currently the state root is hardcoded inside each ROFL source code which looks awful:
https://github.com/oasisprotocol/oasis-sdk/blob/8545062a0a86f0567ce63ac549bada917019dfab/examples/runtime-sdk/rofl-oracle/src/main.rs#L22-L28

Let the Oasis CLI do the detection based on the provided network and ParaTime and store it inside an env variable before compilation, if it's a production build. Otherwise leave undefined. In the example above, a developer would simply read an env variable content if set or use None otherwise.