Fix welcome message again (#280)

`issue_message` seems to be required now, so this just defines a dummy
empty message (the workflow is not triggered for issues anyway). Others
have reported this upstream:
actions/first-interaction#365.

I'm also adding the `repo_token` field which was probably needed all the
time.

Author: msimberg

.github/workflows/welcome.yaml

@@ -14,8 +14,10 @@ jobs:
     steps:
     - uses: actions/first-interaction@v3
       with:
+        repo_token: ${{ secrets.GITHUB_TOKEN }}
         pr_message: |
           Thank you for your contribution to eth-cscs/cscs-docs.
 
           If you have not done so already, please take some time to get familiar with the [contributing guidelines](https://docs.cscs.ch/contributing/).
           Following the guidelines helps us keep the documentation consistent and as useful as possible for users.
+        issue_message: ''