Add pull_request trigger to testing-vars workflow

Author: lystopad

.github/workflows/testing-vars.yml

@@ -15,6 +15,7 @@ env:
   LABEL_DESCRIPTION: "[docker image built on a last commit id from the main branch] Erigon is an implementation of Ethereum (execution layer with embeddable consensus layer), on the efficiency frontier. Archive Node by default."
 
 on:
+  pull_request:
   push:
     branches:
       - 'main'