ci: Fix a minor issue in prepare-artifacts.sh script

ci: Fix a minor issue in prepare-artifacts.sh script

bash is whitespace sensitive.
https://github.com/koalaman/shellcheck/wiki/SC1020

This was noticed from the logs of a CI job
https://gitlab.freedesktop.org/mesa/mesa/-/jobs/13158779
There was no fallout from this bug as not having this variable defined
leads to the right behavior, and defining it to 1 leads to a error
which consequently also leads to the right behaviour.

Signed-off-by: Rohan Garg <rohan.garg@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12562>