fix: optionally use nix only on linux

Author: brynblack

.github/workflows/library-js-staging.yml

@@ -129,7 +129,7 @@ jobs:
               npm run bench --if-present
     steps:
       - uses: actions/checkout@v4
-      - uses: MatrixAI/.github/.github/actions/install-nix@feature-actions
+      - uses: ${{ matrix.platform == 'linux' && 'MatrixAI/.github/.github/actions/install-nix@feature-actions' || null }}
       - uses: actions/download-artifact@v4
         with:
           name: dist