Always use arm64 standard runners

[llucax]

What's needed?

When arm64 runners were introduced, standard runners only worked for public repos, so for private repos one had to create their how custom runners.

This introduced some complexity in the CI workflow:

frequenz-repo-config-python/cookiecutter/{{cookiecutter.github_repo_name}}/.github/workflows/ci.yaml

Line 77 in fe49b18

runs-on: ${{ matrix.os }}${{ matrix.arch != 'amd64' && format('-{0}', matrix.arch) || '' }}

Now GitHub made available the standard arm64 for private repos too.

Proposed solution

Use the new standard arm64 runners always by removing the complicated construction of the runs-on.

Use cases

No response

Alternatives and workarounds

No response

Additional context

No response

[llucax]

The expression there is actually not related to using a different runner for private and public repos, it is just a consequence of a more complicated than necessary matrix definition.

Using a different runner for private repositories was actually never completed (#413), and we are simplifying the matrix in #511.