build(deps): bump DeterminateSystems/update-flake-lock from 20 to 21

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](https://github.com/determinatesystems/update-flake-lock/compare/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] <support@github.com>

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

@@ -13,7 +13,7 @@ jobs:
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@v10
       - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v20
+        uses: DeterminateSystems/update-flake-lock@v21
         with:
           pr-title: "Update flake.lock" # Title of PR to be created
           pr-labels: |                  # Labels to be set on the PR