gnupg module: build custom pinentry for specified flavor only

[SCOTT-HAMILTON]

Motivation for this change

Fixes #124753

Things done

option programs.gnupg.agent.pinentryFlavor will build a custom pinentry with only the needed flavor available as I think this was intented (and pinentry is quite quick to build anyway).

• 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 cf the linked issue above
• 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/)
• Added a release notes entry if the change is major or breaking
• Fits CONTRIBUTING.md.

Pinging maintainers :

@fpletz @bkchr @shosti

[roberth]

It's not specific to this use of pinentry. All pinentry variants suffer from this kind of fragility. #133156

[fpletz]

I think something like #133542 is the better fix for this issue.