build(deps): 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.yml

@@ -39,7 +39,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v14
+        uses: DeterminateSystems/nix-installer-action@v15
       - name: Setup Nix cache
         uses: DeterminateSystems/magic-nix-cache-action@main
       - name: Nix Flake Check
@@ -54,7 +54,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v14
+        uses: DeterminateSystems/nix-installer-action@v15
       - name: Setup Nix cache
         uses: DeterminateSystems/magic-nix-cache-action@main
       - name: Nix Build