ci: 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](DeterminateSystems/nix-installer-action@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>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ${{ matrix.runs-on }}
     steps:
       - uses: actions/checkout@v4
-      - uses: DeterminateSystems/nix-installer-action@v14
+      - uses: DeterminateSystems/nix-installer-action@v15
       - uses: DeterminateSystems/magic-nix-cache-action@v8
       - name: Run `nixci build`
         run: |
@@ -44,7 +44,7 @@ jobs:
       - uses: actions/checkout@v4
         with:
           submodules: recursive
-      - uses: DeterminateSystems/nix-installer-action@v14
+      - uses: DeterminateSystems/nix-installer-action@v15
       - uses: DeterminateSystems/magic-nix-cache-action@v8
       - name: Run Zola
         run: |
@@ -104,7 +104,7 @@ jobs:
         with:
           fetch-depth: 0
       - uses: Swatinem/rust-cache@v2
-      - uses: DeterminateSystems/nix-installer-action@v14
+      - uses: DeterminateSystems/nix-installer-action@v15
       - uses: DeterminateSystems/magic-nix-cache-action@v8
       - uses: actions-rs/cargo@v1
         with: