fix(ci): avoid relative path flake input in examples

The CI job failed during `nix flake check --no-build` with:

  error: cannot fetch input 'path:../.?lastModified=0&narHash=...'
  because it uses a relative path

This comes from examples/flake.nix using a relative flake input:
  rustfs-flake.url = "path:../.";

Relative path-based flake inputs are rejected in CI evaluation contexts.
Switch the example to use a GitHub flake URL so `nix flake check` and `nix eval`
work reliably on GitHub Actions.

Changes:
- examples/flake.nix: replace `rustfs-flake.url = "path:../.";` with
  `rustfs-flake.url = "github:rustfs/rustfs-flake?ref=main";`

Refs:
- Failing job: https://github.com/rustfs/rustfs-flake/actions/runs/24138134845/job/70431678489

Author: houseme

.github/workflows/ci.yml

@@ -51,7 +51,8 @@ jobs:
       - name: Evaluate example configuration
         run: |
           cd examples
-          nix eval .#nixosConfigurations.example-host.config.services.rustfs.enable
+          nix eval --override-input rustfs-flake "path:${GITHUB_WORKSPACE}" \
+          .#nixosConfigurations.example-host.config.services.rustfs.enable
 
   build-linux-package:
     name: Build Linux package

examples/flake.nix

@@ -17,7 +17,7 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    rustfs-flake.url = "path:../.";
+    rustfs-flake.url = "github:rustfs/rustfs-flake?ref=main";
   };
 
   outputs = { self, nixpkgs, rustfs-flake }: {