fix: remove nix flake check from CI

nix flake check evaluates all flake outputs across all systems,
causing cross-platform build failures (e.g. aarch64-linux derivations
on x86_64-linux runners). The per-host build step already validates
each configuration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: stackptr

.github/workflows/ci.yml

@@ -37,7 +37,6 @@ jobs:
         nix profile install --inputs-from . attic#attic-client
         attic login rc https://cache.zx.dev ${{ secrets.ATTIC_TOKEN }}
         attic use rc:main
-    - run: nix flake check
     - name: Build system configuration
       run: |
         if [ "${{ matrix.system }}" = "aarch64-darwin" ]; then