Fix: Avoid requiring -i to propagate output

[lolgab]

This removes the following drawback:

• when compiling things in non-interactive mode, the output of Scala CLI, that prints errors and warnings, is sometimes trapped - use of ./mill -i is recommended, which slows Mill commands a bit

[alexarchambault]

Thanks @lolgab, that's neat!

[alexarchambault]

By the way, I'm phasing out mill-scala-cli from the projects I used from (Scala CLI, coursier, maybe Almond too), as it seems recent changes in Mill fixed the incremental issues I was seeing. So I'm not sure who uses mill-scala-cli any more… Or what the project future will be…

[lolgab]

I was reading the incremental compilation issue description in Mill and was amused looking at how easy to switch the compile task to use a completely different toolchain. If the problem is fixed upstream is even better. This project can become a showcase of Mill's customization options.