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]>

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

@@ -31,7 +31,7 @@ jobs:
           name: klarkc
           authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
       - name: Update ${{ matrix.template }} flake.lock
-        uses: DeterminateSystems/update-flake-lock@v20
+        uses: DeterminateSystems/update-flake-lock@v21
         with:
           path-to-flake-dir: ${{ matrix.template }}
           commit-msg: "build(${{ matrix.template }}): Bump flake.lock dependencies"