Bump actions/cache from 5 to 6 (#55)

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](actions/cache@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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
       - name: checking out lambdapi repo ...
         uses: actions/checkout@v7
       - name: recovering cached opam files ...
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: ~/.opam
           key: ${{ runner.os }}-ocaml-${{ matrix.ocaml-version }}