Allow runner selection for our CI workflowss

As expressed by @skatsaounis in https://github.com/canonical/observability/pull/210, it could be a good idea to allow an easy way to override which runners are used, so that workflows are more easily usable by others.

We should tackle this after the new versioned CI is published.

Start by reviewing pull request 210 and the versioned CI workflows referenced in the issue. Determine how runner selection can be overridden across those workflows; the work is done when users can select alternative runners through the supported configuration without changing each workflow's implementation.

Written by the indexing model from the issue text.