CI: update Python to 3.12

It seems 3.8 is no longer available.

And use a newer version of the setup-python action.

Author: ctrueden

.github/workflows/build.yml

@@ -18,8 +18,8 @@ jobs:
         with:
            ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}
       - name: Set up Python
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v5
         with:
-          python-version: 3.8
+          python-version: 3.12
       - name: Execute the build
         run: .github/build.sh