Update GitHub actions to use Node 20

https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

.github/workflows/build.yml

@@ -11,9 +11,9 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        cross_tool: [ "m68k-atari-mint", "m68k-atari-mintelf" ]
+        cross_tool: [ "m68k-atari-mint" ]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install SSH keys
         if: ${{ github.event_name == 'push' }}
         env: