Bump DeterminateSystems/magic-nix-cache-action from 4 to 6

Bumps [DeterminateSystems/magic-nix-cache-action](https://github.com/determinatesystems/magic-nix-cache-action) from 4 to 6.
- [Release notes](https://github.com/determinatesystems/magic-nix-cache-action/releases)
- [Commits](DeterminateSystems/magic-nix-cache-action@v4...v6)

---
updated-dependencies:
- dependency-name: DeterminateSystems/magic-nix-cache-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
  steps:
  - uses: actions/checkout@v4
  - uses: DeterminateSystems/nix-installer-action@v10
- - uses: DeterminateSystems/magic-nix-cache-action@v4
+ - uses: DeterminateSystems/magic-nix-cache-action@v6
 
  # Evaluate the devshell here so that the time reported for subsequent
  # steps that use it reflect what is actually done there.
@@ -45,7 +45,7 @@ jobs:
  steps:
  - uses: actions/checkout@v4
  - uses: DeterminateSystems/nix-installer-action@v10
- - uses: DeterminateSystems/magic-nix-cache-action@v4
+ - uses: DeterminateSystems/magic-nix-cache-action@v6
 
  - name: Build
  run: nix build --print-build-logs