linux-firmware: update to current Git head

Update the linux-firmware package in order to force the buildbots to fetch the
proper mirrored version.

Currently each builder has its own copy of the linux-firmware checkout staged
in its own dl/, since the package was updated before the mirrored copy has
been uploaded. The builders then subsequently uploaded their own copy instead,
leading to md5sum mismatches since each clone produces different tarballs.

By bumping the package to a new version and uploading the mirrored archive
with the proper md5sum beforehand, the builders will fetch that instead and
not upload their own copies.

To properly solve that problem in the future we need to ensure that packed
checkouts become reproducable.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>