build(deps): bump actions/download-artifact from 5 to 6

Bumps [actions/download-artifact](https://github.com/actions/download-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/download-artifact/releases)
- [Commits](https://github.com/actions/download-artifact/compare/v5...v6)

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

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

.github/workflows/buildkit.yml

@@ -343,7 +343,7 @@ jobs:
     steps:
       -
         name: Download artifacts
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           path: ${{ env.DESTDIR }}
           pattern: buildkit-*

.github/workflows/dockerd.yml

@@ -114,7 +114,7 @@ jobs:
           buildkitd-flags: --debug
       -
         name: Download dockerd
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: dockerd
           path: ./build/

.github/workflows/test-os.yml

@@ -144,7 +144,7 @@ jobs:
           cache: false
       -
         name: Download artifacts
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: buildkit-windows-amd64
           path: ${{ github.workspace }}\bin
@@ -211,7 +211,7 @@ jobs:
         uses: actions/checkout@v5
       -
         name: Download artifacts
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: buildkit-freebsd-amd64
           path: ${{ env.DESTDIR }}