chore(integration-tests): change build.rs to pull submodules (#1723)

### 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](https://github.com/privacy-scaling-explorations/zkevm-circuits/blob/main/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](https://github.com/privacy-scaling-explorations/zkevm-circuits/blob/main/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.

<hr>

### PR code explanation

This PR contains:
- Adding new error kind to `BuildError` enum:
```rust
/// 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](https://github.com/privacy-scaling-explorations/zkevm-circuits/blob/main/integration-tests/run.sh#L94):
```rust
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()))?;
```

Author: curryrasul

Diff for: integration-tests/build.rs

@@ -27,6 +27,9 @@ pub struct CompiledContract {
 
 #[derive(Error, Debug)]
 enum BuildError {
+    /// Failed to pull OZ submodule from github
+    #[error("FailedToPullSubmodule({0:})")]
+    FailedToPullSubmodule(String),
     /// Failed to detect solidity compiler or compiler version.
     #[error("FailedToGetSolidity({0:})")]
     FailedToGetSolidity(String),
@@ -100,6 +103,19 @@ const BINDINGS_DR: &str = "src";
 fn main() -> Result<(), BuildError> {
     println!("cargo:rerun-if-changed=build.rs");
 
+    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()))?;
+
     let solc: Solc = Solc::default();
     let _solc_version = solc
         .version()