Bump DeterminateSystems/nix-installer-action from 14 to 15

Bumps [DeterminateSystems/nix-installer-action](https://github.com/determinatesystems/nix-installer-action) from 14 to 15.
- [Release notes](https://github.com/determinatesystems/nix-installer-action/releases)
- [Commits](https://github.com/determinatesystems/nix-installer-action/compare/v14...v15)

---
updated-dependencies:
- dependency-name: DeterminateSystems/nix-installer-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/flake-update.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-      - uses: DeterminateSystems/nix-installer-action@v14
+      - uses: DeterminateSystems/nix-installer-action@v15
       - uses: DeterminateSystems/magic-nix-cache-action@v8
       - uses: DeterminateSystems/update-flake-lock@v24
         with: