deploy: allow deploying the Function manually

When forking GitGitGadget's GitHub App, GitHub Actions are turned off by
default. Allow deploying the App in such circumstances by adding a
workflow dispatch trigger.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/deploy.yml

@@ -1,6 +1,7 @@
 name: Deploy to Azure
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main