Add condition on "deploy" step to only trigger on forks

see DISCLAIMER comment
add required secrets in comment

.github/workflows/build_and_deploy.yml

@@ -39,10 +39,23 @@ jobs:
           path: ./build/blog
 
   # Deployment job: heavily inspired from https://swharden.com/blog/2022-03-20-github-actions-hugo/
-  # /!\ only triggers on push events
+  # /!\ only triggers on push events AND non-fork repos
+  ## Required secrets:
+  #   - SSH_KNOWN_HOSTS
+  #   - PRIVATE_SSH_KEY
+  #   - CI_USER_NAME
+  #   - STATIC_WEBSITE_PATH
   deploy:
     needs: [build]
-    if: ${{ github.event_name == 'push' }}
+    # DISCLAIMER:
+    # The following is a very POOR solution to avoid *failing deploy step* due to missing secrets
+    # on fork repositories, but sadly the `env` context is not accessible from `jobs.<job_id>.if`:
+    # https://docs.github.com/en/actions/learn-github-actions/contexts#context-availability
+    #
+    # If for any reason you want to trigger this step on your fork remove the following line or
+    # open an issue https://github.com/iScsc/blog.iscsc.fr/issues, we'll find a better way to skip
+    # this step.
+    if: ${{ github.event_name == 'push' && github.event.repository.fork == 'false' }}
     runs-on: ubuntu-latest
     steps:
       - name: 🛠️ Setup build directory