deploy: allow deploying in forks

It would be nice if we could deploy the Azure Function contingent on the
presence of the `AZURE_CLIENT_ID` secret. However, this is not possible
in GitHub workflows: the job-level `if:` conditions lack access to the
`secrets` context. Strangely enough, they _do_ have access to the `vars`
context...

To successfully deploy the Azure Function, it needs to know which
`gitgitgadget-workflows` fork to target when triggering workflow runs,
anyway, so let's _require_ a repository variable called
`DEPLOY_WITH_WORKFLOWS` that specifies that fork in the form
`<org>/gitgitgadget-workflows`.

Note that such a fork _must_ have the `CONFIG` repository variable that
contains the corresponding project configuration; The `deploy` workflow
will retrieve this configuration and overwrite
`gitgitgadget-config.json` with it, augmenting the `workflowsRepo`
information on the fly.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/deploy.yml

@@ -15,11 +15,24 @@ permissions:
 
 jobs:
   deploy:
-    if: github.event.repository.fork == false
+    if: github.event.repository.fork == false || vars.DEPLOY_WITH_WORKFLOWS != ''
     environment: deploy-to-azure
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
+      - name: retrieve `vars.CONFIG` from workflows repo
+        if: vars.DEPLOY_WITH_WORKFLOWS != ''
+        env:
+          WORKFLOWS_REPO: '${{ vars.DEPLOY_WITH_WORKFLOWS }}'
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          WORKFLOWS_OWNER="${WORKFLOWS_REPO%%/*}" &&
+          WORKFLOWS_NAME="${WORKFLOWS_REPO#*/}" &&
+          gh variable get CONFIG --repo "$WORKFLOWS_REPO" |
+          jq '. + { "workflowsRepo": { "owner": "'"$WORKFLOWS_OWNER"'", "name": "'"$WORKFLOWS_NAME"'" } }' \
+            >GitGitGadget/gitgitgadget-config.json &&
+          echo "Using the following configuration:" &&
+          cat GitGitGadget/gitgitgadget-config.json
       - name: 'Login via Azure CLI'
         uses: azure/login@v2
         with: