more robust;

--- a/src/Pure/System/components.scala	Fri Dec 07 12:25:31 2018 +0100
+++ b/src/Pure/System/components.scala	Fri Dec 07 12:31:37 2018 +0100
@@ -36,7 +36,8 @@
   {
     val name = Archive.get_name(archive.file_name)
     progress.echo("Unpacking " + name)
-    Isabelle_System.gnutar("-C " + File.bash_path(dir) + " -xzf " + File.bash_path(archive)).check
+    Isabelle_System.gnutar(
+      "-C " + File.bash_path(dir) + " -xzf " + File.bash_path(archive.absolute)).check
     name
   }