ffsend: 0.2.36 -> 0.2.38

[lilyball]

Motivation for this change

Also add support for xclip on Linux.

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 (Docker using nixos/nix image)
• 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 nox --run "nox-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.

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Tested both Darwin and (using Docker) Linux. The Linux version was tested without an X11 server, but I got the expected "Error: Can't open display: (null)" output from xclip when running ./result/bin/ffsend u -c foo so it seems to have invoked xclip correctly.

[dywedir]

Thanks!