Adjust logback outfile file upon startup

runner/resources/logback.xml

@@ -1,6 +1,6 @@
 <configuration debug="false" scan="false">
   <appender name="FILE" class="ch.qos.logback.core.FileAppender">
-    <file>out/mill.log</file>
+    <file>${mill.log.file:-out/mill.log}</file>
       <encoder>
         <pattern>%date %level [%thread] %logger{36} %msg%n</pattern>
       </encoder>

runner/src/mill/runner/MillMain.scala

@@ -38,6 +38,7 @@ object MillMain {
   }
 
   def main(args: Array[String]): Unit = SystemStreams.withTopLevelSystemStreamProxy {
+    adjustLogbackOutputFile()
     val initialSystemStreams = SystemStreams.original
     // setup streams
     val (runnerStreams, cleanupStreams, bspLog) =
@@ -436,6 +437,10 @@ object MillMain {
     for ((k, v) <- desiredProps) System.setProperty(k, v)
   }
 
+  def adjustLogbackOutputFile(): Unit =
+    if (!sys.props.contains("mill.log.file"))
+      sys.props("mill.log.file") = s"${OutFiles.out}/mill.log"
+
   def withOutLock[T](
       noBuildLock: Boolean,
       noWaitForBuildLock: Boolean,

runner/src/mill/runner/MillServerMain.scala

@@ -23,6 +23,8 @@ object MillServerMain {
       }
     )
 
+    MillMain.adjustLogbackOutputFile()
+
     val acceptTimeoutMillis =
       Try(System.getProperty("mill.server_timeout").toInt).getOrElse(30 * 60 * 1000) // 30 minutes