mkarchiso: do not try to use an non existent GPG public key file

The bootstrap build mode never calls _export_gpg_publickey, so even if the GPG key is passed with the -g option and thus the gpg_key variable is set, the ${work_dir}/pubkey.gpg file will not exist. This has not caused any issue so far because the ARCHISO_GNUPG_FD file descriptor opens the file for both reading and writing, which means the file gets created if it does not exist.

Assign the exported public key file name to a gpg_publickey variable in _export_gpg_publickey and check for it when the file is used.

Since the exist status of the gpg command cannot be checked, look for the exported public key file instead.