Use windows agent for build, variable for publish

This should allow us to reuse the oss template for publish too, and tweak per-repo as needed.

Author: kzu

.github/workflows/os-matrix.json

@@ -0,0 +1 @@
+["windows-latest"]

.github/workflows/publish.yml

@@ -18,7 +18,7 @@ env:
 
 jobs:
   publish:
-    runs-on: ubuntu-latest
+    runs-on: ${{ variables.publish.agent || 'ubuntu-latest' }}
     steps:
       - name: 🤘 checkout
         uses: actions/checkout@v4