proper src1, amending 20157c8ab3f3;

--- a/src/Pure/System/isabelle_system.scala	Sat Feb 27 19:51:56 2021 +0100
+++ b/src/Pure/System/isabelle_system.scala	Sat Feb 27 20:49:38 2021 +0100
@@ -231,7 +231,7 @@
   {
     val src1 = src.expand
     val src1_dir = src1.dir
-    if (!src.starts_basic) error("Illegal path specification " + src1 + " beyond base directory")
+    if (!src1.starts_basic) error("Illegal path specification " + src1 + " beyond base directory")
     copy_file(base_dir + src1, Isabelle_System.make_directory(target_dir + src1_dir))
   }