CI: let Dependabot track GitHub Actions versions (#693)

Dependabot only watched vcpkg, so the action pins drifted by hand:
windows-build.yml sat on actions/checkout@v4 while everything else moved to
v6. Add the github-actions ecosystem and bump that stray checkout to v6.

Signed-off-by: Xavier Roche <roche@httrack.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.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