fix: remove turborepo binary dependency on ffi (vercel#3683)

Currently our release process is broken due to `protoc` not being setup
on the machine:
https://github.com/vercel/turbo/actions/runs/4118079447/jobs/7110161689#step:6:363

We shouldn't need protoc when building the top level Rust binary as it's
only used when building `turborepo-ffi` which is used by `go-turbo` to
call Rust code. `turborepo-ffi` is already built if one runs `make
go-turbo` which happens as part of our Rust binary's build script (when
not creating a release build). During a release build we don't build
`go-turbo` since it is built in an earlier step and is only needed when
we package up the two binaries.

We could instead setup `protoc` in the Rust build process, but that adds
time to the release process.

crates/turborepo/Cargo.toml

@@ -12,10 +12,6 @@ rustls-tls = ["turborepo-lib/rustls-tls"]
 [build-dependencies]
 build-target = "0.4.0"
 
-# we do not actually depend on this, but need
-# to build it before our build.rs script is run
-turborepo-ffi = { path = "../turborepo-ffi" }
-
 [dev-dependencies]
 assert_cmd = "2.0.7"
 itertools = "0.10.5"