Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/desktop-nightly.yml

@@ -60,7 +60,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Restore Rust build cache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: |
             ~/.cargo/registry