Run examples with local repo under out/ rather than ~/.ivy2/local

[alexarchambault]

This changes how Mill is run in integration tests and examples. This makes it stop publishing things under ~/.ivy2/local, and use something like out/dist/localRepo.dest instead, which is the Task.dest of the new dist.localRepo task. That new repository is a local Maven repositiory rather than an Ivy one, which has a slightly different file structure, the same as Maven Central.

For that new repository to be taken into account, the Mill launcher used in examples and ITs is now a script (bat under Windows, bash everywhere else) that sets the COURSIER_REPOSITORIES env var, so that the new local repo is added to it, and gets automatically used when running examples and ITs.

The second commit of this PR mainly updates the file names of the locally published Mill artifacts, because of the switch from an Ivy repository to a Maven one.

That's been useful to me when using a locally published Mill version, to develop on Mill or other projects. Running examples or ITs erases it, which is annoying, but also somewhat unexpected (running tests shouldn't erase things in the local repository, at least not things put there manually on purpose). This PR fixes that.

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This adds back #5949 that was reverted in #6067, and changes dist.installLocalCache, so that it publishes things under ~/.ivy2/local like dist.installLocal does (rather than using the local repo under out/, which is more likely to go away).

[lefou]

Can you please include the full change description of the original PR? (Otherwise git archeology is getting harder.)