chore: build-system submodule=>subrepo

[ludamad]

Description

Let's move build-system to a subrepo. The tradeoffs for submodules are worse.
Subrepo has some edge cases but for me they're now well understood. The major thing is it optimizes for the normal case by being just normal files plus a metadata file. Even if subrepo messes up sync'ing to the upstream repo, aztec keeps its base truth moving and we can sync later.

Pros over submodules:

• No one will pull and have an out of sync build-system, very common accident is to push a build-system revert right now
• The mirror action is already done and I just point it to build-system and nothing should be lost

Cons:

• No automatic two-way mirroring. We can manually recover (or even implement this) but for simplicity let's just work on build-system from aztec where everyone can see it.

Do we still want the other repo: I say default yes as long as it's basically free. We can punt folding it in if it causes pain

[spalladino]

This means we no longer have any submodules that point to repos owned by us in the system, right? Hooray!

[ludamad]

There's technically one minor holdout as barretenberg still has build-system, but only used once mirrored in barretenberg repo. I'll keep an eye out there depending if we actually ever do need to iterate on CI from the barretenberg repo or if we just delete it eventually.