zynq: try to work around hydra restricted mode problems

zynq.nix

@@ -2,8 +2,6 @@ let
   pkgs = import <nixpkgs> {};
   zynq-rs-latest = import <zynq-rs>;
   artiq-zynq = import <artiq-zynq>;
-  zynq-rs = import "${<artiq-zynq>}/zynq-rs.nix";
-  zc706-szl = (import zynq-rs).zc706-szl;
   artiq-fast = import <artiq-fast> { inherit pkgs; };
 in
   (
@@ -23,7 +21,10 @@ in
         touch $out
         '';
     });
-    zc706-hitl-tests = pkgs.lib.hydraJob (pkgs.stdenv.mkDerivation {
+    zc706-hitl-tests = pkgs.lib.hydraJob (let
+      zynq-rs = import "${<artiq-zynq>}/zynq-rs.nix";
+      zc706-szl = (import zynq-rs).zc706-szl;
+    in pkgs.stdenv.mkDerivation {
       name = "zc706-hitl-tests";
 
       # requires patched Nix