nix: Add --store flag

This is a bit user-friendlier than using $NIX_REMOTE.

src/nix/command.cc

@@ -57,9 +57,16 @@ bool MultiCommand::processArgs(const Strings & args, bool finish)
         return Args::processArgs(args, finish);
 }
 
+StoreCommand::StoreCommand()
+{
+    storeUri = getEnv("NIX_REMOTE");
+
+    mkFlag(0, "store", "store-uri", "URI of the Nix store to use", &storeUri);
+}
+
 void StoreCommand::run()
 {
-    run(openStore());
+    run(openStoreAt(storeUri));
 }
 
 }

src/nix/command.hh

@@ -18,9 +18,8 @@ class Store;
 /* A command that require a Nix store. */
 struct StoreCommand : virtual Command
 {
-    bool reserveSpace;
-    StoreCommand(bool reserveSpace = true)
-        : reserveSpace(reserveSpace) { };
+    std::string storeUri;
+    StoreCommand();
     void run() override;
     virtual void run(ref<Store>) = 0;
 };