The forthcoming 1.3.0 release will be signed with my personal GnuPG key; the
installation script needs to tell users how to fetch it.
* etc/guix-install.sh (OPENPGP_SIGNING_KEY_ID): Remove variable.
(GPG_SIGNING_KEYS): New associative array.
(chk_gpg_keyring): Process all the keys contained in the above array.
(main) <GUIX_BINARY_FILE_NAME>: Double quote variable.