fixed system tag.

tests/testsystem.jet

@@ -0,0 +1,8 @@
+#!../jet-2.0
+<jet name1="localname" filterblanklines="true" trimlines="true">
+<set name="abc" value="abcdefg" />
+<system cmd="cat" status="stat" input="$[abc]xyz" name="test1" />
+test1=$[test1]
+name1=#[localname]
+status=$[stat]
+</jet>