artiq-full: concatenate paths as paths not strings

fixes running with restrict-eval=true on Hydra

artiq-full.nix

@@ -13,7 +13,7 @@ let
         );
       isStandalone = jsonFile:
         (builtins.fromJSON (
-          builtins.readFile "${sinaraSystemsSrc}/${jsonFile}"
+          builtins.readFile (<sinaraSystemsSrc> + "/${jsonFile}")
         )).base == "standalone";
     in
       map (builtins.replaceStrings [".json"] [""]) (

artiq-full/artiq-board.nix

@@ -16,7 +16,7 @@ let
   cargoDeps = fetchcargo rec {
     name = "artiq-firmware-cargo-deps";
     src = "${artiqSrc}/artiq/firmware";
-    sha256 = (import "${artiqSrc}/artiq/firmware/cargosha256.nix");
+    sha256 = import (artiqSrc + "/artiq/firmware/cargosha256.nix");
   };
 
   cargoVendored = pkgs.stdenv.mkDerivation {