Bump julia-actions/setup-julia from 2.3.0 to 2.6.0 (#814)

Bumps [julia-actions/setup-julia](https://github.com/julia-actions/setup-julia) from 2.3.0 to 2.6.0.
- [Release notes](https://github.com/julia-actions/setup-julia/releases)
- [Changelog](https://github.com/julia-actions/setup-julia/blob/master/devdocs/making_a_new_release.md)
- [Commits](julia-actions/setup-julia@780022b...9b79636)

---
updated-dependencies:
- dependency-name: julia-actions/setup-julia
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

.github/workflows/exercise-tests.yml

@@ -28,7 +28,7 @@ jobs:
     steps:
       - uses: actions/checkout@eef61447b9ff4aafe5dcd4e0bbf5d482be7e7871
 
-      - uses: julia-actions/setup-julia@780022b48dfc0c2c6b94cfee6a9284850107d037
+      - uses: julia-actions/setup-julia@9b79636afcfb07ab02c256cede01fe2db6ba808c
         with:
           version: ${{ matrix.julia-version }}