chore: fix PR deployment previews (#2114)

Adds missing .env.preview values.

Signed-off-by: Philipp Rudloff <philipp.rudloff@konghq.com>

.env.preview

@@ -1 +1,4 @@
 VITE_ZONE_CREATION_FLOW=enabled
+# The following are templating placeholders for the index.html file. They’re going to be replaced using server-side templating when serving the GUI application in production.
+VITE_BASE_GUI_PATH=/gui
+VITE_KUMA_CONFIG='{"baseGuiPath":"/gui","apiUrl":"http://localhost:5681","version":"2.4.0","product":"Kuma","mode":"global","environment":"universal","storeType":"postgres","apiReadOnly":false}'