Bump DeterminateSystems/update-flake-lock from 19 to 20

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>

.github/workflows/flake-update.yml

@@ -16,7 +16,7 @@ jobs:
           fetch-depth: 0
       - uses: DeterminateSystems/nix-installer-action@v4
       - uses: DeterminateSystems/magic-nix-cache-action@v2
-      - uses: DeterminateSystems/update-flake-lock@v19
+      - uses: DeterminateSystems/update-flake-lock@v20
         with:
           pr-title: "chore: update flake.lock"
           # Labels to be set on the PR