v.help: use os.executable() instead of @vexe as an anchor, so `v help…

…` will work more robustly.

With that change, `v help` will always work, even right after a V archive is downloaded,
and extracted in an arbitrary folder on another machine, without needing `v self` first.

vlib/v/help/help.v

@@ -8,7 +8,7 @@ const (
 
 // print_and_exit Prints the help topic and exits
 pub fn print_and_exit(topic string) {
-	vexe := @VEXE
+	vexe := os.executable()
 	vroot := os.dir(vexe)
 	topicdir := os.join_path(vroot, 'vlib', 'v', 'help')