.gitlab/ci/build_releng.sh: get rid of the ephemeral PGP key
Now that https://ipxe.archlinux.org/releng/netboot/archlinux.ipxe does not use the GPG based root file system image verification anymore ( https://github.com/archlinux/archweb/pull/436 ), there is no need to sign the root file system image with GPG.
OpenSSL CMS signing & verification still remains and it is a lot simpler to use.