better fix with explanation of sbt issue

rocket/chisel-dependent.sbt

@@ -1,8 +1,10 @@
 // Provide a managed dependency on chisel if -DchiselVersion="" is
 // supplied on the command line.
 
-lazy val chiselVersion = System.getProperty("chiselVersion", "None")
+val chiselVersion_r = System.getProperty("chiselVersion", "None")
 
-libraryDependencies ++= ( if (chiselVersion != "None" ) (
-    "edu.berkeley.cs" %% "chisel" % chiselVersion
+// _r a temporary fix until sbt 13.6 https://github.com/sbt/sbt/issues/1465
+
+libraryDependencies ++= ( if (chiselVersion_r != "None" ) (
+    "edu.berkeley.cs" %% "chisel" % chiselVersion_r
 ) :: Nil; else Nil)