exactly. I am surprised by the amount of comments in this and similar threads praying for github to add more complexity to an already over-complicated and fragile solution.
Because you may want caching, or to install certain software in the VM or container that you already have locally. Separating scripts could help, but IME the CI environment is always different enough that some branching or mimicking is needed to harmonize them.
Caching is fine, you likely don’t have to replicate that locally as in theory it should do nothing. For installing software, create a docker image that already has everything you need.