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)
diff --git a/src/version.c b/src/version.c
index 0deebee..6c2d63c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    680,
+/**/
     679,
 /**/
     678,