Pass env parameter to os.SubProcess.env in MillMain

[lolgab]

Fixes #2881 since now correct env is used all the time.

Also updates os-lib to 0.10.5

Reproduction

// build.sc
import mill._
import mill.scalalib._

object root extends RootModule with ScalaModule {
  def scalaVersion = "3.3.1"
}

// src/main.scala
@main
def run =
  println(s"[app] MY_ENV=${sys.env.get("MY_ENV")}")

Run the following command:

rm -rf out; MY_ENV=hello mill run; mill run

Before this PR it outputs:

rm -rf out; MY_ENV=hello mill run; mill run
[build.sc] [48/52] compile 
Compiling compiler interface...
[info] compiling 1 Scala source to /Users/lorenzo/scala/repro/out/mill-build/compile.dest/classes ...
[info] done compiling
[35/48] compile 
[info] compiling 1 Scala source to /Users/lorenzo/scala/repro/out/compile.dest/classes ...
[info] done compiling
[48/48] run 
[app] MY_ENV=Some(hello)
[48/48] run 
[app] MY_ENV=Some(hello)

After this PR it outputs:

rm -rf out; MY_ENV=hello mill run; mill run
[build.sc] [48/52] compile 
Compiling compiler interface...
[info] compiling 1 Scala source to /Users/lorenzo/scala/repro/out/mill-build/compile.dest/classes ...
[info] done compiling
[35/48] compile 
[info] compiling 1 Scala source to /Users/lorenzo/scala/repro/out/compile.dest/classes ...
[info] done compiling
[48/48] run 
[app] MY_ENV=Some(hello)
[48/48] run 
[app] MY_ENV=None

[lihaoyi]

Looks good to me. @lolgab could you update the PR description with how this was tested? Manual testing is fine, just need to write down how you did it in case we need to reproduce the tests later