more informative release_snapshot, to see better where the cronjob fails;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Fri Jun 10 14:36:05 2022 +0200
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Fri Jun 10 15:34:25 2022 +0200
@@ -18,6 +18,7 @@
   val backup = "lxbroy10:cronjob"
   val main_dir: Path = Path.explode("~/cronjob")
   val main_state_file: Path = main_dir + Path.explode("run/main.state")
+  val build_release_log: Path = main_dir + Path.explode("run/build_release.log")
   val current_log: Path = main_dir + Path.explode("run/main.log")  // owned by log service
   val cumulative_log: Path = main_dir + Path.explode("log/main.log")  // owned by log service
 
@@ -81,8 +82,11 @@
   /* build release */
 
   val build_release: Logger_Task =
-    Logger_Task("build_release",
-      { logger => Isabelle_Devel.release_snapshot(logger.options, get_rev(), get_afp_rev()) })
+    Logger_Task("build_release", { logger =>
+      build_release_log.file.delete
+      Isabelle_Devel.release_snapshot(logger.options, get_rev(), get_afp_rev(),
+        progress = new File_Progress(build_release_log))
+    })
 
 
   /* remote build_history */

--- a/src/Pure/Admin/isabelle_devel.scala	Fri Jun 10 14:36:05 2022 +0200
+++ b/src/Pure/Admin/isabelle_devel.scala	Fri Jun 10 15:34:25 2022 +0200
@@ -32,11 +32,13 @@
 
   /* release snapshot */
 
-  def release_snapshot(options: Options, rev: String, afp_rev: String): Unit = {
+  def release_snapshot(options: Options, rev: String, afp_rev: String,
+    progress: Progress = new Progress
+  ): Unit = {
     Isabelle_System.with_tmp_dir("isadist") { target_dir =>
       Isabelle_System.update_directory(root + Path.explode(RELEASE_SNAPSHOT),
         { website_dir =>
-          val context = Build_Release.Release_Context(target_dir)
+          val context = Build_Release.Release_Context(target_dir, progress = progress)
           Build_Release.build_release_archive(context, rev)
           Build_Release.build_release(options, context, afp_rev = afp_rev,
             build_sessions = List(Isabelle_System.getenv("ISABELLE_LOGIC")),