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](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>

Author: dependabot[bot]

action.yml

@@ -80,7 +80,7 @@ runs:
 
   # Optionally share Eldev's GPAC between workflow executions.
   - if:   inputs.cache == 'true'
-    uses: actions/cache@v5
+    uses: actions/cache@v6
     with:
       # FIXME: Might want to cache the whole of `~/.cache/eldev'.  However, that would
       #        include the installed package of Eldev itself, so we'd need to be careful