patch 8.0.0680: plugins in start packages are sourced twice

Problem:    Plugins in start packages are sourced twice. (mseplowitz)
Solution:   Use the unmodified runtime path when loading plugins (test by Ingo
            Karkat, closes #1801)

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    680,
 /**/
     679,
 /**/