Review and potentially fix `TaskCtx.env` handling when on Windows

[lefou]

On Windows, environment variables don't need to be cases-sensitive, hence a simple lookup for a key might not be enough.

For example the scala.sys.env map has a withDefault override that tried for find the variable more vigorous. Task.ctx.get("UserPROFILE") should return a value, even is there is only a key for UserProfile`.

See also this review comment thread: https://github.com/com-lihaoyi/mill/pull/4917#pullrequestreview-2793944318