add const variable $current_file for script files

script here refers to the entrypoint file when running scripts with nu
like `nu script.nu`

crates/nu-cli/src/eval_file.rs

@@ -74,6 +74,18 @@ pub fn evaluate_file(
         .expect("internal error: missing filename");
 
     let mut working_set = StateWorkingSet::new(engine_state);
+
+    let current_file = working_set.add_variable(
+        b"$current_file".into(),
+        Span::unknown(),
+        nu_protocol::Type::String,
+        false,
+    );
+    working_set.set_variable_const_val(
+        current_file,
+        Value::string(file_path.to_string_lossy(), Span::unknown()),
+    );
+
     trace!("parsing file: {}", file_path_str);
     let block = parse(&mut working_set, Some(file_path_str), &file, false);