Do not include the gpg binary unless ARCHISO_GNUPG_FD is set durring build

Handle the condition when signature verification is requested but the gpg binary is not available.

Since now there is OpenSSL CMS based root file system image verification, GPG based verification should be made optional. mkarchiso currently requires running as root, which complicates signing with user's GPG keys. E.g. running it with sudo may require chowning the TTY and passing the GNUPGHOME environment variable. While the cause of this annoyance is mkarchiso, gpg doesn't make the process easy. OpenSSL CMS based signing has none of these issues.