github-actions: 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](DeterminateSystems/update-flake-lock@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] <[email protected]>

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

@@ -37,7 +37,7 @@ jobs:
 
       - id: update_nix_flake
         name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v19
+        uses: DeterminateSystems/update-flake-lock@v20
         with:
           sign-commits: false
           gpg-fingerprint: ${{ secrets.GPG_FINGERPRINT }}