nix: enable allowFilter in knot module

Signed-off-by: oppiliappan <[e-mail]>

nix/modules/knot.nix

···
               email = [e-mail]
           [receive]
               advertisePushOptions = true
+          [uploadpack]
+              allowFilter = true
           EOF
           ${setMotd}
           chown -R ${cfg.gitUser}:${cfg.gitUser} "${cfg.stateDir}"