docs(release): Add Nix flake install instructions to release notes

Adds a Nix Flake section to the release body template alongside the
existing curl, binary download, and container image options. Shows
both `nix run` for one-shot execution and the Flox manifest entry
for cross-repo consumption, both pinned to the released tag.

Author: edenreich

.releaserc.yaml

@@ -130,6 +130,21 @@ plugins:
         curl -fsSL https://raw.githubusercontent.com/inference-gateway/cli/main/install.sh | bash -s -- --install-dir $HOME/.local/bin
         ```
 
+        ### Nix Flake
+
+        Run directly without installing:
+
+        ```bash
+        nix run github:inference-gateway/cli/<%= nextRelease.gitTag %>
+        ```
+
+        Or pin it in a [Flox](https://flox.dev) manifest (`.flox/env/manifest.toml`):
+
+        ```toml
+        [install]
+        infer.flake = "github:inference-gateway/cli/<%= nextRelease.gitTag %>"
+        ```
+
         ### Binary Download
 
         Download the appropriate binary for your platform from the assets below.