nix upgrade-nix: make the source URL an option

This new option enables organizations to more easily manage their Nix
fleet's deployment, and ensure a consistent and planned rollout of Nix
upgrades.

src/libstore/globals.hh

@@ -1084,6 +1084,16 @@ public:
         true, // document default
         Xp::ConfigurableImpureEnv
     };
+
+    Setting<std::string> upgradeNixStorePathUrl{
+        this,
+        "https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix",
+        "upgrade-nix-store-path-url",
+        R"(
+          Used by `nix upgrade-nix`, the URL of the file that contains the
+          store paths of the latest Nix release.
+        )"
+    };
 };