Bump actions/cache from 3 to 4 in the github-actions group (#1300)

Bumps the github-actions group with 1 update: [actions/cache](https://github.com/actions/cache).


Updates `actions/cache` from 3 to 4
- [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/v3...v4)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/test.yaml

@@ -69,7 +69,7 @@ jobs:
         activate-environment: pypsa-eur
 
     - name: Cache Conda env
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       with:
         path: ${{ env.CONDA }}/envs
         key: conda-${{ runner.os }}--${{ runner.arch }}--${{ env.today }}-${{ hashFiles('envs/environment.yaml') }}