lxc-generator: alpine: signature files are downloaded instead of rootfs

Apparently, alpine started to publish .asc signatures for non-edge release versions. The generate-lxc.sh script will now select the signature file instead of the rootfs archive.

lxc-create then (correctly) complains that the downloaded rootfs is not a tar archive. For the longest time I suspected that some of the tooling is wrong, only to later discover that indeed it's not a tar 😄

I'll try to email a MR to fix that or post the patch here if that fails.