ci: bump actions/cache from 4 to 5

Bumps [actions/cache](https://github.com/actions/cache) from 4 to 5.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v4...v5)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
       run: |
         mastersha=$(gh api repos/plumed/plumed2/branches --paginate --jq '.[] | select( .name | match("^master$")) | .commit.sha')
         echo "key=$mastersha" >> $GITHUB_OUTPUT
-    - uses: actions/cache@v4
+    - uses: actions/cache@v5
       with:
         path: |
           ~/.ccache