all: Add --sim.buildarg flag

[marioevz]

Adds --sim.buildarg as a parameter to add build arguments to the simulator Dockerfile.

[marioevz]

@spencer-tb @danceratopz

[danceratopz]

@KolbyML @ScottyPoi This change will allow us to specify the "test fixture" release version used in the ethereum/execution-spec-test simulators on the command-line, just as we already specify the client version under test. Just wanted to share: perhaps it's useful for the Portal simulators?

[KolbyML]

This change will allow us to specify the "test fixture" release version used in the ethereum/execution-spec-test simulators on the command-line, just as we already specify the client version under test. Just wanted to share: perhaps it's useful for the Portal simulators?

I would find this ability useful, since I want/plan to be able to sub in testnet test data for our portal simulators.