ci: bump DeterminateSystems/update-flake-lock from 19 to 20 (#57)

Bumps [DeterminateSystems/update-flake-lock](https://github.com/determinatesystems/update-flake-lock) from 19 to 20.
- [Release notes](https://github.com/determinatesystems/update-flake-lock/releases)
- [Commits](https://github.com/determinatesystems/update-flake-lock/compare/v19...v20)

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

.github/workflows/update.yml

@@ -17,7 +17,7 @@ jobs:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
       - name: "flake: update inputs"
-        uses: DeterminateSystems/update-flake-lock@v19
+        uses: DeterminateSystems/update-flake-lock@v20
         with:
           pr-title: "flake: update inputs"
           pr-labels: dependencies