Make make_tarball vars writable to allow multiple kernel vers

[elcabaye]

Without this change, when passing multiple -k to dkms mktarball, only the first one would be added to the tarball.

[evelikov]

Ouch, thanks for catching this. Seems like I miss-read the manual - -k (and -a) can be passed multiple times.

Can I ask you to add a simple test? IIRC CI setup has single kernel, which is fine - one can pass the same -k version/arch argument twice.

Thanks in advance

[scaronni]

Closing as it has been posted again for review as the original proposer is MIA.