ci: bump DeterminateSystems/nix-installer-action from 14 to 15 (#497)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/ci.yaml

@@ -31,7 +31,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v14
+        uses: DeterminateSystems/nix-installer-action@v15
         with:
           diagnostic-endpoint: ""
 

.github/workflows/update-lock.yaml

@@ -37,7 +37,7 @@ jobs:
           token: ${{ steps.app-token.outputs.token }}
 
       - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@v14
+        uses: DeterminateSystems/nix-installer-action@v15
         with:
           diagnostic-endpoint: ""