Log command to be run in Jvm.callProcess when debugging is enabled (com-lihaoyi#4792)

(Context is I'm currently having issues running specific tests on the
main branch, and I was hoping `--debug` would help me, but it doesn't 😅)

Author: alexarchambault

main/util/src/mill/util/Jvm.scala

@@ -77,7 +77,7 @@ object Jvm {
       shutdownGracePeriod: Long = 100,
       destroyOnExit: Boolean = true,
       check: Boolean = true
-  ): CommandResult = {
+  )(implicit ctx: Ctx): CommandResult = {
     val cp = cpPassingJarPath match {
       case Some(passingJarPath) if classPath.nonEmpty =>
         createClasspathPassingJar(passingJarPath, classPath.toSeq)
@@ -96,6 +96,10 @@ object Jvm {
 
     if (cwd != null) os.makeDir.all(cwd)
 
+    ctx.log.debug(
+      s"Running ${commandArgs.map(arg => "'" + arg.replace("'", "'\"'\"'") + "'").mkString(" ")}"
+    )
+
     val processResult = os.proc(commandArgs)
       .call(
         cwd = cwd,