fix(pr): test-deploy not getting variables in PR to fork source

.github/workflows/test-deploy.yml

@@ -36,10 +36,19 @@ jobs:
         run: .github/workflows/excluded-paths.sh
 
       - name: Test build website
-        # We also create the CNAME file, this is useless in staging,
+        # When running this test in a PR from the staging to the production repository,
+        # vars.* variables are not made avaiable to the action !?!
+        # c.f: see https://github.com/orgs/community/discussions/44322
+        # We are thus faking those by sourcing .env.sample file
+        # thus using hopefully harmless staging values
+        #
+        # We also create the CNAME file, which is useless in staging,
         # but required in production.
-        # It is created temporarily in the static/ folder, because
-        # docusaurus will copy it to /build
+        # It is created temporarily in the static/ folder, and docusaurus will
+        # copy it to the /build directory
         run: |
+          if [ -z "$DTT_DOCS_URL" ] || [ -z "DTT_DOCS_BASE_PATH" ]; then
+            source .env.sample
+          fi
           echo "$DTT_DOCS_URL" | cut -f 3 -d/ | tee static/CNAME
           npm run build