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](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]>

Author: dependabot[bot]

.github/workflows/update_nix.yml

@@ -18,7 +18,7 @@ jobs:
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
 
       - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v20
+        uses: DeterminateSystems/update-flake-lock@v21
         with:
           pr-title: "Weekly PR to bump flake.nix" # Title of PR to be created
           pr-labels: |                  # Labels to be set on the PR