.gitlab/ci/build_archiso.sh: improve CI codesigning certificate

Adjust subject name to more closely match what's used in create_ephemeral_pgp_key.

Reduce the certificate validity to two days. These are just temporary certificates, they will not be used anywhere.

.gitlab/ci/build_archiso.sh: create a valid code signing certificate

Make sure the certificate has a extendedKeyUsage section with codeSigning per the iPXE requirements.