Add a workflow to delete a self-hosted runner VM

Since we're using ephemeral runners, the VMs are no longer used after
they completed a workflow job, and need to be deleted. This GitHub
workflow is designed to perform that action, and it is intended to be
triggered by the GitForWindowsHelper GitHub App in response to receiving
a `workflow_job.completed` event.

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

Author: dscho

.github/workflows/delete-self-hosted-runner.yml

@@ -0,0 +1,35 @@
+name: delete azure-self-hosted-runner
+run-name: Delete ${{ inputs.runner_name }}
+
+on:
+  workflow_dispatch:
+    inputs:
+      runner_name:
+        type: string
+        required: true
+        description: The name of the runner that needs to be deleted
+
+env:
+  ACTIONS_RUNNER_NAME: ${{ github.event.inputs.runner_name }}
+
+# The following secrets are required for this workflow to run:
+# AZURE_CREDENTIALS - Credentials for the Azure CLI. It's recommended to set up a resource
+#                     group specifically for self-hosted Actions Runners.
+#   az ad sp create-for-rbac --name "{YOUR_DESCRIPTIVE_NAME_HERE}" --role contributor \
+#     --scopes /subscriptions/{SUBSCRIPTION_ID_HERE}/resourceGroups/{RESOURCE_GROUP_HERE} \
+#     --sdk-auth
+# AZURE_RESOURCE_GROUP - Resource group to create the runner(s) in
+jobs:
+  delete-runner:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Azure Login
+      uses: azure/login@v1
+      with:
+        creds: ${{ secrets.AZURE_CREDENTIALS }}
+    - name: Delete VM '${{ env.ACTIONS_RUNNER_NAME }}'
+      uses: azure/CLI@v1
+      with:
+        azcliversion: 2.43.0
+        inlineScript: |
+          az vm delete -n "$ACTIONS_RUNNER_NAME" -g ${{ secrets.AZURE_RESOURCE_GROUP }} --yes