thunderbird: allow overwriting wrapped binary

  # For that to work out of the box, it requires `gnupg` on PATH and

  # `gpgme` in `LD_LIBRARY_PATH`; we do this below.

  buildCommand = old.buildCommand + ''

    wrapProgram $out/bin/thunderbird \

    wrapProgram $out/bin/${browser.binaryName} \

      --prefix LD_LIBRARY_PATH ':' "${lib.makeLibraryPath [ gpgme ]}" \

      --prefix PATH ':' "${lib.makeBinPath [ gnupg ]}"

  '';