ci: bump DeterminateSystems/update-flake-lock from 14 to 15

Bumps [DeterminateSystems/update-flake-lock](https://github.com/DeterminateSystems/update-flake-lock) from 14 to 15.
- [Release notes](https://github.com/DeterminateSystems/update-flake-lock/releases)
- [Commits](https://github.com/DeterminateSystems/update-flake-lock/compare/v14...v15)

---
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/update-flake.yml

@@ -14,7 +14,7 @@ jobs:
       - name: Install Nix
         uses: cachix/install-nix-action@v18
       - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v14
+        uses: DeterminateSystems/update-flake-lock@v15
         with:
           token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
           pr-labels: dependencies