sdrangel: set SOAPYSDR_DIR in build

[numinit]

Motivation for this change

sdrangel has some strange logic for finding SoapySDR in its cmake script. If cmake locates soapysdr, the UI components for soapysdr will be compiled but the plugins won't be. Setting SOAPYSDR_DIR makes sure the plugins are compiled too.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[numinit]

Confirmed working end to end with a local SoapySDR server.