build script: Compress qcow2 images by default unless --no-compress-qcow is set. This streamlines the process to create redistributable qcow2 images. (Before they had to be manually compressed by the builder.) Uploading them uncompressed is not advisable, because the rsync upload tool seems not to understand sparse files. So upload it would upload the apparent file size instead of the actual file size. The --no-compress-qcow might be useful for builders who want to use these images for their own without uploading them anywhere, saves them one step unnecessarily decompressing them.
output
fix