Update DeterminateSystems/nix-installer-action action to v21

Author: renovate[bot]

.github/workflows/pull.yml

@@ -34,7 +34,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/flake-checker-action@3164002371bc90729c68af0e24d5aacf20d7c9f6 # v12
         if: matrix.runner != 'macos-15-intel'
-      - uses: DeterminateSystems/nix-installer-action@v17
+      - uses: DeterminateSystems/nix-installer-action@c5a866b6ab867e88becbed4467b93592bce69f8a # v21
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
       - uses: DeterminateSystems/magic-nix-cache-action@565684385bcd71bad329742eefe8d12f2e765b39 # v13