CI Linux: prevent mkappimage auto upload

mkappimage now tries to automatically upload the asset if it detects
running in GitHub CI, which is not the one that we perhaps want.

See following failed run:
https://github.com/CESNET/UltraGrid/actions/runs/5575039257/jobs/10185765784

.github/workflows/ccpp.yml

@@ -112,7 +112,7 @@ jobs:
     - name: check libc/libstdc++ ABI
       run: .github/scripts/Linux/check_abi.sh 2.31 3.4.28 1.3.12 bin/* lib/ultragrid/*
     - name: Create AppImage
-      run: data/scripts/Linux-AppImage/create-appimage.sh https://github.com/$GITHUB_REPOSITORY/releases/download/continuous/UltraGrid-$CHANNEL-x86_64.AppImage.zsync
+      run: GITHUB_TOKEN= data/scripts/Linux-AppImage/create-appimage.sh https://github.com/$GITHUB_REPOSITORY/releases/download/continuous/UltraGrid-$CHANNEL-x86_64.AppImage.zsync
     - name: Check AppImage
       run: |
         mkdir aitest-context # empty build context