Bump actions/cache from 4 to 6

Bumps [actions/cache](https://github.com/actions/cache) from 4 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](https://github.com/actions/cache/compare/v4...v6)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/dependabot.yml

@@ -6,3 +6,9 @@ updates:
     directory: /src
     schedule:
       interval: weekly
+
+  # Keep the workflow action pins current (they only rot manually otherwise).
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly

.github/workflows/windows-build.yml

@@ -29,7 +29,7 @@ jobs:
     env:
       VCPKG_DEFAULT_BINARY_CACHE: ${{ github.workspace }}\vcpkg_cache
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           submodules: recursive   # coucal lives in src/coucal
 
@@ -60,7 +60,7 @@ jobs:
       # carries the builtin-baseline, so a Dependabot bump busts it; restore-keys
       # still seeds the unchanged ports' archives, so only the bumped one rebuilds.
       - name: Cache vcpkg binary archives
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: ${{ github.workspace }}\vcpkg_cache
           key: vcpkg-${{ matrix.platform }}-${{ hashFiles('src/vcpkg.json') }}