ci: bump actions/cache from 5 to 6

Bumps [actions/cache](https://github.com/actions/cache) from 5 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/v5...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] <[email protected]>

.github/workflows/build.yml

@@ -59,7 +59,7 @@ jobs:
         run: echo "timestamp=`date +%s%N`" >> $GITHUB_OUTPUT
 
       - name: Restore cache
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: ${{ env.CCACHE_DIR }}
           key: ${{ env.TARGET }}-${{ steps.get_time.outputs.timestamp }}
@@ -123,7 +123,7 @@ jobs:
           path: artifact/*
 
       - name: Save Cache
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         if: always()
         with:
           path: ${{ env.CCACHE_DIR }}
@@ -165,7 +165,7 @@ jobs:
           "timestamp=$((Get-Date).Ticks)" >> $env:GITHUB_OUTPUT
 
       - name: Restore cache
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: ${{ env.CCACHE_DIR }}
           key: ${{ matrix.target }}-${{ steps.get_time.outputs.timestamp }}
@@ -261,7 +261,7 @@ jobs:
             build/mpv.pdb
 
       - name: Save Cache
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         if: always()
         with:
           path: ${{ env.CCACHE_DIR }}