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.

[kostko]

This has been solved as part of #335 where the trust root can be obtained automatically and configured as an environment variable. So if the methods are not overriden, these values will be retrieved from the environment variables.