Simplify CI config

[lihaoyi]

Some cleanup in preparation for #4118

Set default --jobs=0.5C in .mill-opts so we don't need to specify it explicitly in CI, and it also works on laptops and other environments with different core counts correctly. TBH could probably go with even less parallelism due to how heavy our integration tests are - with all the compilation and JVM subprocess spawning they can probably easily take up all 4 cores on a Github Actions worker - but I'm leaving it at 0.5C X 4 cores = 2x parallelism in CI for now

Also simplified the windows CI setup: now that we have ./mill.bat in the project root, we can just call ./mill in the windows terminal. I had to delete the .ps1 version because it was taking priority over .bat and failing with the following error:

Run ./mill -i -k __.compile
  
mill.ps1: D:\a\_temp\93d23[8](https://github.com/com-lihaoyi/mill/actions/runs/12292451871/job/34303135448#step:11:8)[8](https://github.com/com-lihaoyi/mill/actions/runs/12292451871/job/34303135448#step:11:9)4-066d-433e-bb26-8539b01a4aee.ps1:2
Line |
   2 |  ./mill -i -k __.compile
     |         ~~
     | Parameter cannot be processed because the parameter name 'i' is ambiguous. Possible matches include:
     | -InformationAction -InformationVariable.
Error: Process completed with exit code 1.

[lefou]

Can we instead move it into a wrapper dir for now? I already archived millw, so it would be unfortunate for users to loose it.

There is the --no-server option that works the same way as -i.