ci(docker): remove build-time nix check and rely on runtime env test

Why:
The Dockerfile previously validated `nix --version` during image build,
which is environment-sensitive and behaves differently in GitHub Actions
vs local/act builds due to PATH and non-login shell differences.

The actual contract is runtime availability of Nix, not build-step PATH
resolution. This is now reliably enforced by the dedicated `test-env-nix`
container test, which validates nix presence and flake execution in the
real execution environment.

This removes flaky CI behavior while keeping stronger, more accurate
coverage of the intended guarantee.

https://chatgpt.com/share/693bfbc7-63d8-800f-9ceb-728c7a58e963

Dockerfile

@@ -52,11 +52,6 @@ RUN set -e; \
     make install; \
     rm -rf /build
 
-# ------------------------------------------------------------
-# Show Nix Version
-# ------------------------------------------------------------
-RUN command -v nix && nix --version
-
 # ------------------------------------------------------------
 # Runtime working directory and dev entrypoint
 # ------------------------------------------------------------