docs(crates/turborepo): add capnp to build dependencies

[aryasaatvik]

Description

I was setting up turborepo locally on macOS. ran into the follow error. installing capnp fixed it

   Compiling turborepo-lockfiles v0.1.0 (turborepo/crates/turborepo-lockfiles)
error: failed to run custom build command for `turborepo-lib v0.1.0 (turborepo/crates/turborepo-lib)`

Caused by:
  process didn't exit successfully: `turborepo/target/debug/build/turborepo-lib-167cf8cfecddd530/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-changed=./src/daemon/proto/turbod.proto
  cargo:rerun-if-changed=./src/daemon/proto

  --- stderr
  thread 'main' panicked at crates/turborepo-lib/build.rs:27:23:
  schema compiler command: Error { kind: Failed, extra: "Failed to execute `capnp --version`: No such file or directory (os error 2). Please verify that version 0.5.2 or higher of the capnp executable is installed on your system. See https://capnproto.org/install.html" }
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...

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[NicholasLYang]

Thanks for the contribution!

[ijjk]

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• approve CI run for commit: 1b3b43f

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