Bump DeterminateSystems/update-flake-lock from 20 to 21 (#272)

Bumps [DeterminateSystems/update-flake-lock](https://github.com/determinatesystems/update-flake-lock) from 20 to 21.
- [Release notes](https://github.com/determinatesystems/update-flake-lock/releases)
- [Commits](DeterminateSystems/update-flake-lock@v20...v21)

---
updated-dependencies:
- dependency-name: DeterminateSystems/update-flake-lock
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/update-flake-lock.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Install nix
         uses: cachix/install-nix-action@v25
       - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v20
+        uses: DeterminateSystems/update-flake-lock@v21
         with:
           token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
           pr-title: "Update flake.lock"