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] <support@github.com>

.github/workflows/ResInsightMac.yml

@@ -59,7 +59,7 @@ jobs:
           pip install -e GrpcInterface/Python[dev]
 
       - name: Restore buildcache tool binary
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: ${{ github.workspace }}/buildcache-tool
           key: ${{ runner.os }}-buildcache-tool-v0.33.0
@@ -94,7 +94,7 @@ jobs:
 
       - name: Cache buildcache contents
         id: cache-buildcache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: ${{ env.BUILDCACHE_DIR }}
           key: ${{ runner.os }}-appleclang-6.7.0-buildcache-v01-${{ steps.current-time.outputs.formattedTime }}

.github/workflows/ResInsightWithCache.yml

@@ -122,7 +122,7 @@ jobs:
 
       - name: Cache Buildcache
         id: cache-buildcache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: ${{ env.BUILDCACHE_DIR }}
           key: ${{ matrix.config.os }}-${{ matrix.config.cc }}-${{ matrix.config.qt-version }}-cache-v03-${{ steps.current-time.outputs.formattedTime }}

.github/workflows/copilot-setup-steps.yml

@@ -110,7 +110,7 @@ jobs:
         run: echo "formattedTime=$(date -u +%Y-%m-%d)" >> $GITHUB_OUTPUT
 
       - name: Restore buildcache
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           # Inline literals (not ${{ env.X }}) so this works under the Copilot
           # agent, whose per-step runner doesn't expose job-level env to