Replace OpenOptions::new with File::options to be readable (#3059)

## Issue Addressed

Closes #3049 

This PR updates widely but this replace is safe as `File::options()` is equivelent to `OpenOptions::new()`.
ref: https://doc.rust-lang.org/stable/src/std/fs.rs.html#378-380

crypto/eth2_keystore/src/keystore.rs

@@ -21,7 +21,7 @@ use scrypt::{
 };
 use serde::{Deserialize, Serialize};
 use sha2::{Digest, Sha256};
-use std::fs::OpenOptions;
+use std::fs::File;
 use std::io::{Read, Write};
 use std::iter::FromIterator;
 use std::path::Path;
@@ -329,7 +329,7 @@ impl Keystore {
 
     /// Instantiates `self` by reading a JSON file at `path`.
     pub fn from_json_file<P: AsRef<Path>>(path: P) -> Result<Self, Error> {
-        OpenOptions::new()
+        File::options()
             .read(true)
             .write(false)
             .create(false)

crypto/eth2_keystore/tests/tests.rs

@@ -7,7 +7,7 @@ use eth2_keystore::{
     json_keystore::{Kdf, Pbkdf2, Prf, Scrypt},
     Error, Keystore, KeystoreBuilder, DKLEN,
 };
-use std::fs::OpenOptions;
+use std::fs::File;
 use tempfile::tempdir;
 
 const GOOD_PASSWORD: &[u8] = &[42, 42, 42];
@@ -55,7 +55,7 @@ fn file() {
     let path = dir.path().join("keystore.json");
 
     let get_file = || {
-        OpenOptions::new()
+        File::options()
             .write(true)
             .read(true)
             .create(true)