[GLIB] Update env var used to disable using flatpak to build

https://bugs.webkit.org/show_bug.cgi?id=259116

Reviewed by Nikolas Zimmermann.

This env var is more clear in its purpose. WKDEV_SDK was used
by another script but was never an ideal choice.

* Tools/Scripts/webkitdirs.pm:
(shouldUseFlatpak):

Canonical link: https://commits.webkit.org/265955@main

Tools/Scripts/webkitdirs.pm

@@ -2459,7 +2459,7 @@ sub shouldUseFlatpak()
         return 0;
     }
 
-    if ((defined $ENV{'WEBKIT_JHBUILD'} && $ENV{'WEBKIT_JHBUILD'}) or defined $ENV{'WKDEV_SDK'}) {
+    if ((defined $ENV{'WEBKIT_JHBUILD'} && $ENV{'WEBKIT_JHBUILD'}) or defined $ENV{'WEBKIT_BUILD_USE_SYSTEM_LIBRARIES'}) {
         return 0;
     }