Allow URLs in the Nix search path

E.g. to install "hello" from the latest Nixpkgs:

  $ nix-build '<nixpkgs>' -A hello -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz

Or to install a specific version of NixOS:

  $ nixos-rebuild switch -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/63def04891a0abc328b1b0b3a78ec02c58f48583.tar.gz

src/nix-instantiate/nix-instantiate.cc

@@ -155,14 +155,16 @@ int main(int argc, char * * argv)
             return true;
         });
 
+        if (evalOnly && !wantsReadWrite)
+            settings.readOnlyMode = true;
+
+        store = openStore();
+
         EvalState state(searchPath);
         state.repair = repair;
 
         Bindings & autoArgs(*evalAutoArgs(state, autoArgs_));
 
-        if (evalOnly && !wantsReadWrite)
-            settings.readOnlyMode = true;
-
         if (attrPaths.empty()) attrPaths.push_back("");
 
         if (findFile) {
@@ -174,8 +176,6 @@ int main(int argc, char * * argv)
             return;
         }
 
-        store = openStore();
-
         if (readStdin) {
             Expr * e = parseStdin(state);
             processExpr(state, attrPaths, parseOnly, strict, autoArgs,