Older (< 6.0) versions of OMNeT++ are not working on macOS/aarch64

[rhornig]

Those versions were meant to run only on x86_64 and currently it is not possible to run opp_env in an emulated x86 shell because NIX itself is always an aarch64 process, so anything started from there also runs as aarch64.

As a first step, we must warn the user and throw an error if they install any project using <6.0 omnet on a macOS/aarch64 machine. (opp_env on native x86_64 machine should work just fine with older versions).

On long term, we may try to force NIX to use x86_64 as the system but we would still have to use an x86_64 build from NIX.

A third option would be to run in NIXLESS mode and manually switch to x86_64 mode (using the arch command) when opp_env executes/starts the build scripts.