Upgrade rust-toolchain to 1.67.1

[eval-exec]

• Upgrade rust-toolchain to 1.67.1
• fix cargo clippy warnings.
• rename Env::default() to Env::default_env() because https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait

error: method `default` can be confused for the standard trait method `std::default::Default::default`
   --> src/db_options.rs:132:5
    |
132 | /     pub fn default() -> Result<Self, Error> {
133 | |         let env = unsafe { ffi::rocksdb_create_default_env() };
134 | |         if env.is_null() {
135 | |             Err(Error::new("Could not create mem env".to_owned()))
...   |
138 | |         }
139 | |     }
    | |_____^
    |
    = help: consider implementing the trait `std::default::Default` or choosing a less ambiguous method name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait
    = note: `-D clippy::should-implement-trait` implied by `-D warnings`

Request for review. @zhangsoledad
Note: libc::size_t is a type alias of usize:
https://github.com/rust-lang/libc/blob/586541b97e047cfec540fad752bc1eaca4acac36/src/unix/mod.rs#L19

[eval-exec]

I rename Env::default() to Env::default_env(), because:

error: method `default` can be confused for the standard trait method `std::default::Default::default`
   --> src/db_options.rs:132:5
    |
132 | /     pub fn default() -> Result<Self, Error> {
133 | |         let env = unsafe { ffi::rocksdb_create_default_env() };
134 | |         if env.is_null() {
135 | |             Err(Error::new("Could not create mem env".to_owned()))
...   |
138 | |         }
139 | |     }
    | |_____^
    |
    = help: consider implementing the trait `std::default::Default` or choosing a less ambiguous method name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait
    = note: `-D clippy::should-implement-trait` implied by `-D warnings`

[eval-exec]

I forget to update rust-toolchains in CI workflows