use a meaningful artifact name to distinguish the builds

[razr]

The artifact name is always the same: RPI4 RT Kernel deb packages. Depending on the parameters we could change the name dynamically to make it more meaningful, e.g. 5.15.35-rt41-raspi-kernel to distinguish the builds

    - run: | 
        echo "artifactName=`ls /home/user/linux_build/linux-image-* | cut -d '-' -f 3-4`-kernel" >> $GITHUB_ENV
      - uses: actions/upload-artifact@v2
        with:
          name: '${{ env.artifactName }}
          path: ~/linux_build/*.deb