Remove the build script and the exported `BUNDLE_CAR`

[Stebalien]

At the moment, you can import the actors bundle via cargo and access the bundle via a compile-time constant. Unfortunately, this means "checking" the root actors crate involves building the entire bundle. And, TBH, the entire system is a bit gnarly. Instead, we should just explicitly build the bundle from src/main.rs and expect users to do the same.

The ref-fvm and actor-utils repos will need to move away from using cargo to pull in the actors and will need to either fetch a bundle from github or build from source explicitly. This shouldn't be super complicated and, importantly, it'll remove the current cyclic dependency between these projects.

[Stebalien]

So... how would people feel about checking in compressed released cars into separate branches in a builtin-actors-releases repo? By separate branches, I mean one branch per car: mainnet/vN etc.

This would let us import bundles into projects as git submodules, specifying shallow = 1 in the submodule definition to only checkout the exact bundle we care about. Compressed (with zstd), each bundle is only ~1.6MiB, so it's not a huge deal as long as we only download a single bundle (which shallow = 1 should let us do).

Maybe a better approach would be to have one branch per network and use tags for each version? But the core idea still stands.

[rvagg]

Not a fan, mostly because submodules are garbage UX. I'd rather have lashings of bash or Makefile content than rely submodules.

[rvagg]

But wrt a separate repo holding the assets, bultin-actors-releases, since we now have reproducibility, I think that would be really neat.

[Stebalien]

Ah, I guess I've customized my global git config with:

[submodule]
	recurse = true
	propagateBranches = true

Which makes them "work by default". TBH, doing this in a makefile is much easier anyways.