Update registries if they already exist

[omus]

While using julia-actions/cache I encountered a scenario where my CI registry didn't contain the latest version of a released package.

Here's a snippet of my workflow:

- uses: julia-actions/cache@v1
   with:
     cache-name: "${{ github.workflow }}-${{ github.job }}-${{ matrix.pkg.name }}-${{ matrix.version }}-${{ matrix.os }}-${{ matrix.arch }}"
     cache-registries: true
     cache-compiled: true
- uses: julia-actions/add-julia-registry@v1

I'll outline the series of events I believed that took place:

1. Opened my PR which triggered the above workflow. The added registries were then cached
2. Updated the Project.toml in my PR to require a release of a package which was not yet released during step 1.
3. Pushed change resulted in triggering of the CI workflow which had a cache hit and loaded the cache registries.
4. The julia-actions/add-julia-registry sees that the registries are present and effectively does nothing.

I believe the add-julia-registry action should update any existing registries to avoid this kind of problem

[omus]

The julia-actions/cache action now uses cache-registries: true by default. From a code deep dive I did any workflow that uses Pkg.instantiate should automatically update the registries: julia-actions/cache#62 (comment)

All that's left to do here is to try and attempt to reproduce the issue with the latest GitHub actions.

[omus]

Fixed in #29