Fix GitHub Actions branch name

I literally copied this from a GitHub repo... wtf.

.github/workflows/upload-secrets.yml

@@ -2,7 +2,7 @@ name: Publish encrypted laptop configuration data
 
 on:
   push:
-    branches: [$default-branch]
+    branches: ['main']
   workflow_dispatch:
 
 # Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages