'cargo miri --color=always' does not work

[RalfJung]

Cargo does not forward the --color argument to rustc and instead interprets it itself. However for cargo-miri that does not work since the relevant output of diagnostics happens when the program is run, not when it is built. So the --color flag basically is currently ignored by cargo miri.

There were some attempts to fix this, but they lead to other issues, so we'd have to find a better strategy.