more robust etc/preferences: default value remains;

--- a/src/Pure/Admin/build_release.scala	Wed Oct 26 15:02:11 2022 +0200
+++ b/src/Pure/Admin/build_release.scala	Wed Oct 26 16:26:23 2022 +0200
@@ -244,13 +244,17 @@
 
           val build_command =
             "bin/isabelle build -o system_heaps -b -- " + Bash.strings(build_sessions)
+          def system_apple(b: Boolean): String =
+            """{ echo "ML_system_apple = """ + b + """" > "$(bin/isabelle getenv -b ISABELLE_HOME_USER)/etc/preferences"; }"""
+
           val build_script =
             List(
               "cd " + File.bash_path(remote_dir),
               "tar -xf tmp.tar",
+              """mkdir -p "$(bin/isabelle getenv -b ISABELLE_HOME_USER)/etc" """,
+              system_apple(false),
               build_command,
-              """mkdir -p "$(bin/isabelle getenv -b ISABELLE_HOME_USER)/etc" """,
-              """{ echo "ML_system_apple = false" > "$(bin/isabelle getenv -b ISABELLE_HOME_USER)/etc/preferences"; }""",
+              system_apple(true),
               build_command,
               "tar -cf tmp.tar heaps")
           ssh.execute(build_script.mkString(" && "), settings = false).check