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/build.yml

@@ -41,7 +41,7 @@ jobs:
         with:
           purge-packages: 'true'
 
-      - uses: DeterminateSystems/nix-installer-action@v14
+      - uses: DeterminateSystems/nix-installer-action@v15
         with:
           source-url: https://install.lix.systems/lix/lix-installer-x86_64-linux
           extra-conf: |

.github/workflows/update.yml

@@ -32,7 +32,7 @@ jobs:
 
       - run: git checkout -B flake-lock
 
-      - uses: DeterminateSystems/nix-installer-action@v14
+      - uses: DeterminateSystems/nix-installer-action@v15
         with:
           source-url: https://install.lix.systems/lix/lix-installer-x86_64-linux
           extra-conf: |
@@ -101,7 +101,7 @@ jobs:
             host: github.com
             private-key: ${{ secrets.SSH_PRIVATE_KEY }}
 
-      - uses: DeterminateSystems/nix-installer-action@v14
+      - uses: DeterminateSystems/nix-installer-action@v15
         with:
           source-url: https://install.lix.systems/lix/lix-installer-x86_64-linux
           extra-conf: |