flake: don't use url literals

Signed-off-by: Yuxuan Shui <[email protected]>

Author: yshui

flake.nix

@@ -1,9 +1,9 @@
 {
   inputs = {
-    flake-utils.url = github:numtide/flake-utils;
-    nixpkgs.url = github:nixos/nixpkgs;
+    flake-utils.url = "github:numtide/flake-utils";
+    nixpkgs.url = "github:nixos/nixpkgs";
     git-ignore-nix = {
-      url = github:hercules-ci/gitignore.nix/master;
+      url = "github:hercules-ci/gitignore.nix/master";
       inputs.nixpkgs.follows = "nixpkgs";
     };
   };