Bump clippy msrv to 1.64

the msrv was bumped in in the Cargo.toml #847 but not in clippy.toml,
clippy was complaining at startup and defaulting to 1.57

tests/depfile.rs

@@ -93,7 +93,6 @@ fn test_project(project_path: &str) {
     );
 
     std::fs::remove_dir_all(build_dir).expect("Failed to remove old build directory");
-    ()
 }
 
 macro_rules! test_file {

tests/tests.rs

@@ -251,7 +251,7 @@ fn run_compile_test(
     );
     if generate_depfile {
         let depfile = depfile_contents.expect("No depfile generated");
-        assert!(depfile.len() > 0);
+        assert!(!depfile.is_empty());
         let mut rules = depfile.split(':');
         let target = rules.next().expect("No target found");
         assert_eq!(target, generated_file.as_os_str().to_str().unwrap());