gha: tests: add workaround for pull_request_target

Fixes #209

Kudos @shcheklein

Author: efiop

.github/workflows/test.yml

@@ -2,6 +2,7 @@ on:
   push:
     branches:
       - master
+  pull_request:
   pull_request_target:
 
 name: Tests
@@ -28,6 +29,9 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+      with:
+	# NOTE: needed for pull_request_target to use PR code
+        ref: ${{ github.event.pull_request.head.sha || github.ref }}
 
     - uses: actions/setup-python@v4
       with: