aarch64 is supported by the plugin since version 3.15.4 according to

https://developers.hp.com/hp-linux-imaging-and-printing/release_notes

I have tested this change and it worked on my raspberry pi and with my
printer.

A minor caveat is that I had to revert to hplip 3.18.5 in order to be
able to build hplip itself for aarch64. The version 3.19.1 currently
fails to build on non-x86 because of

https://bugs.launchpad.net/hplip/+bug/1785230

It is for this reason that I'm not adding aarch64-linux to the platforms
list for hplip in this commit.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.