Release v3.21.5 (#73)

* Prepare release v3.21.5

* Update the state.json for v3.21.5

* Update README.md and action.yml for v3.21.5

* flake.lock: Update

Flake lock file updates:

• Updated input 'nixpkgs':
    'https://api.flakehub.com/f/pinned/NixOS/nixpkgs/0.1.1017464%2Brev-567a49d1913ce81ac6e9582e3553dd90a955875f/019ed27e-44b2-7462-b1ef-3564aa6c28ee/source.tar.gz' (2026-06-16)
  → 'https://api.flakehub.com/f/pinned/NixOS/nixpkgs/0.1.1027867%2Brev-d407951447dcd00442e97087bf374aad70c04cea/019f32d6-f801-7201-b6cb-59773d7eec95/source.tar.gz' (2026-07-05)

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

action.yml

@@ -72,7 +72,7 @@
         "source-tag": {
             "description": "The tag of `nix-installer` to use (conflicts with `source-revision`, `source-branch`, `source-pr`)",
             "required": false,
-            "default": "v3.21.2"
+            "default": "v3.21.5"
         },
         "source-url": {
             "description": "A URL pointing to a `nix-installer` executable",