chore(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](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] <[email protected]>

.github/workflows/build.yml

@@ -144,7 +144,7 @@ jobs:
             .sources
           key: fonts-${{ hashFiles('Makefile-fonts') }}
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v14
+        uses: DeterminateSystems/nix-installer-action@v15
       - name: Cache Nix dependencies
         uses: DeterminateSystems/magic-nix-cache-action@v8
       - name: Setup developer environment

.github/workflows/nix.yml

@@ -16,7 +16,7 @@ jobs:
         with:
           fetch-depth: 0
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v14
+        uses: DeterminateSystems/nix-installer-action@v15
       - name: Cache Nix dependencies
         uses: DeterminateSystems/magic-nix-cache-action@v8
       # Upstream package sometimes has flags set that disable flake checking