refactoring: unwrap preprocessing env set key as its always Some

src/preprocess.rs

@@ -9,14 +9,12 @@ const ENV_SET: Lazy<Regex> = Lazy::new(|| Regex::new(r#"(?P<key>\w+)=(?P<value>\
 /// returns `Ok(None)`, do not execute a command.
 pub fn preprocess(line: String) -> Result<Option<String>, ShellError> {
     if let Some(capture) = ENV_SET.captures(&line) {
-        let Some(key) = capture.name("key") else {
-            return Err(ShellError::MalformedArgs("cannot find key to set env variable".to_string()))
-        };
+        let key = capture.name("key").unwrap().as_str();
         let value = capture
             .name("value")
             .map_or("".to_string(), |v| v.as_str().to_string());
 
-        env::set_var(key.as_str(), value);
+        env::set_var(key, value);
         return Ok(None);
     }