chore(integration-tests): change build.rs to pull submodules

[curryrasul]

Description

Change integration-tests/build.rs to fetch OZ submodule automatically.

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)

Rationale

I propose to change integration-tests/build.rs to automatically fetch OZ contracts submodule, so that initial rust-analyzer indexation works without errors. Right now it's necessary to fetch the submodule manually or by running integration-tests/run.sh.

How Has This Been Tested?

This feature is hard to test neither with unit-tests nor with integration-tests, so it was tested locally, on a new & clean repo and worked without any errors.

Open question

A completed spawned process returns its exit-status/code, so possibly it's a good tone to check if it's successful (exit code = 0); though the command is clear and straightforward - that's why I decided not to do that.

---

PR code explanation

This PR contains:

• Adding new error kind to BuildError enum:

  /// Failed to pull OZ submodule from github
  #[error("FailedToPullSubmodule({0:})")]
  FailedToPullSubmodule(String),

• Process spawn to fetch submodule (the same as in integration-tests/run.sh:

  std::process::Command::new("git")
  .args([
      "submodule",
      "update",
      "--init",
      "--recursive",
      "--checkout",
      "contracts/vendor",
  ])
  .spawn()
  .and_then(|mut child| child.wait())
  .map_err(|err| BuildError::FailedToPullSubmodule(err.to_string()))?;