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)