Revert "artiq-full: move reading standalone flag from sinara-systems to before generatedNix"

hydra-eval-jobs returned exit code 1:
error: --- Error --- hydra-eval-jobs
worker error: error: --- RestrictedPathError --- hydra-eval-jobs
access to path '/nix/store/xq8mnh5rf5v0www2amzvlbf8m6r0ckjv-pm405hs1s21h4p3bq7fg2vcwwc1gqcfv-source/apm.json' is forbidden in restricted mode

This reverts commit b0e041c716.

artiq-full.nix

@@ -2,23 +2,6 @@
 
 let
   sinaraSystemsSrc = <sinaraSystemsSrc>;
-  standaloneVariants =
-    let
-      jsonFiles =
-        builtins.attrNames (
-          pkgs.lib.filterAttrs (name: type:
-            type != "directory" &&
-            builtins.match ".+\\.json" name != null
-          ) (builtins.readDir sinaraSystemsSrc)
-        );
-      isStandalone = jsonFile:
-        (builtins.fromJSON (
-          builtins.readFile "${sinaraSystemsSrc}/${jsonFile}"
-        )).base == "standalone";
-    in
-      map (builtins.replaceStrings [".json"] [""]) (
-        builtins.filter isStandalone jsonFiles
-      );
 
   generatedNix = pkgs.runCommand "generated-nix" { buildInputs = [ pkgs.nix pkgs.git ]; }
     ''
@@ -128,9 +111,6 @@ let
         "wipm7master"
         "wipm7satellite"
       ]);
-      standaloneVariants = [${builtins.concatStringsSep " " (
-        builtins.map (variant: "\"${variant}\"") standaloneVariants
-      )}];
       # Splitting the build process into software+gateware does
       # not work when artiq embeds compiled firmware into generated
       # Vivado input.
@@ -168,7 +148,7 @@ let
               boardBinaries = boardBinaries;
               inherit target variant;
             };
-          } // (pkgs.lib.optionalAttrs (builtins.elem variant standaloneVariants) {
+          } // (pkgs.lib.optionalAttrs ((builtins.fromJSON (builtins.readFile json)).base == "standalone") {
             "device-db-\''${target}-\''${variant}" = pkgs.stdenv.mkDerivation {
               name = "device-db-\''${target}-\''${variant}";
               buildInputs = ddbDeps;