ci: full matrix across architectures and CUPS versions

[Abd002]

Adds the full CI matrix Rohit uses on the other OpenPrinting C repos: 4 architectures
(amd64, arm64, armhf, riscv64) x 3 CUPS releases (distro libcups2-dev, OpenPrinting/cups
@master, OpenPrinting/libcups @master). Only the distro leg is required; the two @master
legs are non-blocking since they track a moving upstream target.

Also includes two small fixes needed to get there:

• c_char is signed on x86_64 but unsigned on arm64, which broke the DNS-SD callbacks
  under the new arm64 legs.
• cups@master has started tagging cups_dest_flags_e the same way CUPS 3 does, so a
  cups2 build against it now needs the same prefixed constant names.