ci: debian: fix missing dist for bpi-r4

Author: frank-w

.github/workflows/debian.yml

@@ -90,7 +90,7 @@ jobs:
         run: |
           #echo -e "skipkerneldownload=1\nkernelfile=${{ env.R4Kernel }}" > sourcefiles_bpi-r4.conf
           echo -e "userpackages=\"${{ inputs.packages }}\"" >> sourcefiles_bpi-r4.conf
-          ./buildimg.sh bpi-r4 ${{ env.DEBIAN_DIST }}
+          ./buildimg.sh bpi-r4 ${{ vars.DEBIAN_DIST }}
 
       - name: Upload to gdrive
         uses: adityak74/google-drive-upload-git-action@main