Set julia-actions/cache to v2 in action

[JoshuaLampert]

Running this action gives the following warning

The following actions use a deprecated Node.js version and will be forced to run on node20:
actions/cache@4d4ae6ae148a43d0fd1eda1800170683e9882738, pyTooling/Actions/with-post
step@adef08d3bdef092282614f3b683897cefae82ee3. For more info: https://github.blog/changelog/2024-03-07-github
actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

because it still uses julia-actions/cache@v1, which ran on node 16. As I understand without updating to node20 (i.e. without updating to julia-actions/cache@v2) this action will not run anymore in the near future.

I think this is considered a breaking change, see for example julia-actions/cache#117.

See also #30.