Fix executable paths on Windows

Author: JD557

justfile

@@ -1,3 +1,4 @@
+set windows-shell := ["powershell.exe", "-NoLogo", "-Command"]
 
 default:
   just --list

project/src/build.runner.scala

@@ -1,3 +1,5 @@
+import java.util.Locale
+
 import scala.concurrent.duration.*
 
 import fs2.*
@@ -22,6 +24,9 @@ sealed trait BuildTool
 class ScalaCli extends BuildTool
 class Mill extends BuildTool
 
+private lazy val isWindows: Boolean =
+  System.getProperty("os.name").toLowerCase(Locale.ENGLISH).contains("windows")
+
 def buildRunner(
     tool: BuildTool,
     linkingTopic: Topic[IO, Unit],
@@ -66,7 +71,7 @@ def buildRunnerScli(
     .flatMap(
       _ =>
         ProcessBuilder(
-          "scala-cli",
+          if isWindows then "scala-cli.bat" else "scala-cli",
           scalaCliArgs
         ).withWorkingDirectory(workDir)
           .spawn[IO]
@@ -135,7 +140,7 @@ def buildRunnerMill(
   ) ++ extraBuildArgs
   // TODO pipe this to stdout so that we can see linker progress / errors.
   val builder = ProcessBuilder(
-    "mill",
+    if isWindows then "mill.bat" else "mill",
     millargs
   ).withWorkingDirectory(workDir)
     .spawn[IO]