clarified message;

--- a/src/Pure/Admin/build_release.scala	Mon Aug 08 13:33:04 2022 +0200
+++ b/src/Pure/Admin/build_release.scala	Mon Aug 08 14:34:09 2022 +0200
@@ -222,17 +222,22 @@
     options: Options,
     platform: Platform.Family.Value,
     build_sessions: List[String],
-    local_dir: Path
+    local_dir: Path,
+    progress: Progress = new Progress,
   ): Unit = {
     val server_option = "build_host_" + platform.toString
+    val server = options.string(server_option)
+    progress.echo("Building heaps " + commas_quote(build_sessions) +
+      " (" + server_option + " = " + quote(server) + ") ...")
+
     val ssh =
-      options.string(server_option) match {
+       server match {
         case "" =>
           if (Platform.family == platform) SSH.Local
           else error("Undefined option " + server_option + ": cannot build heaps")
         case SSH.Target(user, host) =>
           SSH.open_session(options, host = host, user = user)
-        case s => error("Malformed option " + server_option + ": " + quote(s))
+        case _ => error("Malformed option " + server_option + ": " + quote(server))
       }
     try {
       Isabelle_System.with_tmp_file("tmp", ext = "tar") { local_tmp_tar =>
@@ -583,8 +588,7 @@
         // build heaps
 
         if (build_sessions.nonEmpty) {
-          progress.echo("Building heaps " + commas_quote(build_sessions) + " ...")
-          build_heaps(options, platform, build_sessions, isabelle_target)
+          build_heaps(options, platform, build_sessions, isabelle_target, progress = progress)
         }