Add --no-wait-for-build-lock option

Mainly meant to be used in integration tests

main/api/src/mill/api/Logger.scala

@@ -80,13 +80,16 @@ trait Logger extends Closeable {
     finally removePromptLine()
   }
 
-  def waitForLock(lock: Lock): Locked = {
+  def waitForLock(lock: Lock, waitingAllowed: Boolean): Locked = {
     val tryLocked = lock.tryLock()
     if (tryLocked.isLocked())
       tryLocked
-    else {
+    else if (waitingAllowed) {
       info("Another Mill process is running tasks, waiting for it to be done...")
       lock.lock()
+    } else {
+      error("Cannot proceed, another Mill process is running tasks")
+      throw new Exception("Cannot acquire lock on Mill output directory")
     }
   }
 }

runner/src/mill/runner/MillCliConfig.scala

@@ -136,7 +136,13 @@ case class MillCliConfig(
       doc =
         """Evaluate tasks / commands without acquiring an exclusive lock on the Mill output directory"""
     )
-    noBuildLock: Flag = Flag()
+    noBuildLock: Flag = Flag(),
+    @arg(
+      hidden = true,
+      doc =
+        """Do not wait for an exclusive lock on the Mill output directory to evaluate tasks / commands. Fail if waiting for a lock is needed."""
+    )
+    noWaitForBuildLock: Flag = Flag()
 )
 
 import mainargs.ParserForClass

runner/src/mill/runner/MillMain.scala

@@ -243,7 +243,10 @@ object MillMain {
                       )
                       Using.resources(
                         logger,
-                        logger.waitForLock(outLock)
+                        logger.waitForLock(
+                          outLock,
+                          waitingAllowed = !config.noWaitForBuildLock.value
+                        )
                       ) { (_, _) =>
                         new MillBuildBootstrap(
                           projectRoot = WorkspaceRoot.workspaceRoot,