Support --eval-store in nix-instantiate and nix-build

src/libexpr/common-eval-args.cc

@@ -61,6 +61,14 @@ MixEvalArgs::MixEvalArgs()
             fetchers::overrideRegistry(from.input, to.input, extraAttrs);
         }}
     });
+
+    addFlag({
+        .longName = "eval-store",
+        .description = "The Nix store to use for evaluations.",
+        .category = category,
+        .labels = {"store-url"},
+        .handler = {&evalStoreUrl},
+    });
 }
 
 Bindings * MixEvalArgs::getAutoArgs(EvalState & state)

src/libexpr/common-eval-args.hh

@@ -16,8 +16,9 @@ struct MixEvalArgs : virtual Args
 
     Strings searchPath;
 
-private:
+    std::optional<std::string> evalStoreUrl;
 
+private:
     std::map<std::string, std::string> autoArgs;
 };