Update actions/download-artifact action to v6

Author: renovate[bot]

.github/workflows/nightly.yml

@@ -43,7 +43,7 @@ jobs:
       permissions: write-all
       runs-on: ubuntu-latest
       steps:
-      - uses: actions/download-artifact@v4.3.0
+      - uses: actions/download-artifact@v6.0.0
         with:
           pattern: AppImage-*
           merge-multiple: true

.github/workflows/stable.yml

@@ -43,7 +43,7 @@ jobs:
     permissions: write-all
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/download-artifact@v4.3.0
+      - uses: actions/download-artifact@v6.0.0
         with:
           pattern: AppImage-*
           merge-multiple: true