Migrate to Mill 1.1.2 (was 1.0.6)

[Gedochao]

https://github.com/com-lihaoyi/mill/releases/tag/1.1.2
Here we go again...

• had to change all .mill.scala files to .mill (.mill.scala got dropped in Mill 1.1.x 😕)
• export sub-command now returns build.mill rather than build.mill.scala for all Mill 1.x versions (used to be build.mill.scala for 1.0.x)
• a lot of reflect-config changes for GraalVM's sake, which smh only started breaking with this Mill bump
• some Windows compat stuff, which also started failing here, randomly
• Scala 2 nightly repo times out more often than not, tagged relevant tests as flaky