gnupg: use config flags to find lib prefix instead of their binary tools

[illegalprime]

Motivation for this change

we need to explicitly set these tool's prefixes for cross-compilation to work, otherwise the builder will try to call binaries of these packages to get that information, (which cannot be done since those tools are cross-compiled)

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 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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[peti]

@GrahamcOfBorg build gnupg22