REGTESTS: Require the version 2.2 to execute lua/set_var

This script depends on LUA changes introduced in HAProxy 2.2 and not backported.
diff --git a/reg-tests/lua/set_var.vtc b/reg-tests/lua/set_var.vtc
index 5d37fcb..fc21b41 100644
--- a/reg-tests/lua/set_var.vtc
+++ b/reg-tests/lua/set_var.vtc
@@ -1,4 +1,5 @@
 varnishtest "Lua: set_var"
+#REQUIRE_VERSION=2.2
 #REQUIRE_OPTIONS=LUA
 
 feature ignore_unknown_macro