Various Fixed for Windows

[lefou]

Various fixes only shown on Windows, probably also elsewhere.

• Fixed string stream writer/reader used for mill client-server communication, by using an explicit charset for byte conversion
• Avoid java.nio.file.Paths.get(url.getFile()) in favour of java.nio.file.Paths.get(url.toURI()) as the latter handles windows paths with leading drive letters well
• Improved CI setup for Windows
• Better run/spawn of Java sub-processes by selecting the same Java executable which is used by mill

We still have some CI failures on Windows, as we need some of these fixed in mill before we can benefit from it. Expect more PRs.

Probably related issues:

• Make mill -i to default under Windows OS #781
• Mill Client-Daemon mode hanging in WSL2 Ubuntu 18.04 #874
• Mill is stuck on windows with no output #646
• Mill fail to connect to the server with apple silicon M1 #1098