Create an env variable for mock mode

[rootulp]

Context

There exists this env variable

celestia-zkevm-ibc-demo/.env.example

Lines 1 to 2 in 0add0c9

	# SP1_PROVER={network|local|mock}
	SP1_PROVER=mock

Problem

We'll have mock circuits and real circuits. We need a way to signal to the system which circuits to use

Proposal

Either re-use the existing env variable or create a new env variable. If mock mode is enabled, we'll need to create/set keys that are distinct from the real keys.