Most of our build scripts are sourced rather than executed in a
subshell, so they inherit our environment. Allow our firmware script to
do the same by not exiting when we have no firmware to download, as is
the case on arm32.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31781>