bazaar: enable sftp transport (#21132)

SFTP transport needs python paramiko package, which is now an optional
input

pkgs/applications/version-management/bazaar/default.nix

···
-{ stdenv, fetchurl, python2Packages }:
+{ stdenv, fetchurl, python2Packages
+, withSFTP ? true
+ }:
 
 python2Packages.buildPythonApplication rec {
   version = "2.7";
···
   };
 
   doCheck = false;
+
+  propagatedBuildInputs = []
+  ++ stdenv.lib.optionals withSFTP [ python2Packages.paramiko ];
 
   # Bazaar can't find the certificates alone
   patches = [ ./add_certificates.patch ];