ci: bump DeterminateSystems/nix-installer-action from 14 to 15 (#140)

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/update.yaml

@@ -13,7 +13,7 @@ jobs:
       - name: Checkout repository
         uses: actions/[email protected]
       - name: Install nix
-        uses: DeterminateSystems/nix-installer-action@v14
+        uses: DeterminateSystems/nix-installer-action@v15
         with:
           extra-conf: |
             extra-substituters = https://cache.garnix.io