Misc CI improvements and refactors

[VeckoTheGecko]

I was chatting with Justus yesterday, and he mentioned improvements we can make to our CI setup. I've also wrapped a couple of my own in.

Changes

• Update should-skip-ci to should-run-ci , failing if we shouldn't (which implicitly skips the rest of the workflows)
• Update workflow graph to a chain (rather than directly depending on should-skip-ci and cache-pixi-lock)
• Update the should-run-ci and cache-pixi-lock to be run using slim instances (faster startup and lower resources)

[VeckoTheGecko]

• Update workflow graph to a chain

Before

Now the graph is

[erikvansebille]

Looks good!