Fix CI

[mihaimaruseac]

• [x] Split Haskell cache into 2 components, one for dependencies, one for code
• [x] Reduce recompilation between tests and main library
• [x] Don't run test twice
• [x] Remove PointerType as it was removed from upstream LLVM
• [x] Add tag to all reusable actions so that tools like Dependabot could keep them up to date
• [x] Only run CI on PRs when they are opened or updated

I'm thinking we might not need the --fforce-recomp flag, but for that I'll have to look deeper. For now, https://github.com/mihaimaruseac/mlir-hs/actions/runs/6765678946/job/18385656313 is a green run of the actions, the large time is spent in compiling LLVM, Haskell compilation is quite fast.