Add two more files to tip release

[Alexey Palazhchenko]

• Checksums file
• File with a name like 2021-04-15T12:13:14Z.txt to indicate that builds are going

golang-tip/.github/workflows/build.yml

Lines 117 to 131 in bd6b3b8

       - name: Download artifacts 
         uses: actions/download-artifact@v2.0.9 
  
       - name: List artifacts 
         run: ls -lR 
  
       - name: Publish artifacts 
         uses: marvinpinto/action-automatic-releases@v1.1.1 
         with: 
           repo_token: ${{ secrets.GITHUB_TOKEN }} 
           automatic_release_tag: tip 
           prerelease: false 
           title: tip 
           files: | 
             **.tar.gz