Use new cli library for mixins

[carolynvs]

I'm working on a new library in porter to simplify creating a mixin or plugin. You can see the start of it in #2216. One nice feature in that PR is pkg/cli, which standardizes all the useful setup that we have in porter's main, and allows the mixins and plugins to get that same functionality without having to copy/paste it all.

Once that package is available in porter v1, we should use that for all the mixins (ideally making a mixin library just like I'm doing for plugins, but we can start with this package). When we switch to using pkg/cli, that will give us helper functions to load porter's configuration, and more easily bind flags like PORTER_DEBUG.

In my PR for #1429, I am keeping PORTER_DEBUG around for the mixins, but it would be great if we can use this and start having the mixins load porter's configuration, and then be able to use the same logic for "are we in debug mode" in porter, the plugins AND the mixins. This would also make it possible to do tracing from inside the mixins, since we could start passing opentelemetry environment variables and have them bound when the config is loaded.

[carolynvs]

After I cut 1.0.0 releases for the mixins, we can make this swap. Right now the mixins are all in RC and this is too big of a change for that.