add multi-arch (arm & x64) image
add multiple pipelines (ci now creates the "default" pipeline as a shorthand)
simplify devenv flake input
merge all cache options together, now $NIX_CI_CACHE_STRATEGY decides how the cache works
setup_nix_ci and finalize_nix_ci are now flake packages and work standalone
the specific image is not needed anymore, any image with the right dependencies works
runner cache is not the default anymore (because it sucked most of the time)
the pipeline is selected by $NIX_CI_PIPELINE_NAME or if empty by $CI_PIPELINE_SOURCE,
so for the old behaviour $NIX_CI_PIPELINE_NAME=default is needed, future
work will be needed to handle this more nicely
