genpolicy: fix clippy v1.78.0 warnings

cargo clippy has two new warnings that need addressing:
- assigning_clones
  These were fixed by clippy itself.
- suspicious_open_options
  I added truncate(false) because we're opening the file for reading.

Signed-off-by: Markus Rudy <mr@edgeless.systems>

src/tools/genpolicy/src/mount_and_storage.rs

@@ -57,8 +57,8 @@ pub fn get_policy_mounts(
                 .find(|m| m.destination.eq(&s_mount.destination))
             {
                 // Update an already existing mount.
-                policy_mount.type_ = mount.type_.clone();
-                policy_mount.source = mount.source.clone();
+                policy_mount.type_.clone_from(&mount.type_);
+                policy_mount.source.clone_from(&mount.source);
                 policy_mount.options = mount.options.iter().map(String::from).collect();
             } else {
                 // Add a new mount.
@@ -94,7 +94,7 @@ fn keep_settings_mount(
 fn adjust_termination_path(mount: &mut policy::KataMount, yaml_container: &pod::Container) {
     if mount.destination == "/dev/termination-log" {
         if let Some(path) = &yaml_container.terminationMessagePath {
-            mount.destination = path.clone();
+            mount.destination.clone_from(path);
         }
     }
 }

src/tools/genpolicy/src/policy.rs

@@ -570,10 +570,12 @@ impl AgentPolicy {
         linux.Namespaces = get_kata_namespaces(is_pause_container, use_host_network);
 
         if !c_settings.Linux.MaskedPaths.is_empty() {
-            linux.MaskedPaths = c_settings.Linux.MaskedPaths.clone();
+            linux.MaskedPaths.clone_from(&c_settings.Linux.MaskedPaths);
         }
         if !c_settings.Linux.ReadonlyPaths.is_empty() {
-            linux.ReadonlyPaths = c_settings.Linux.ReadonlyPaths.clone();
+            linux
+                .ReadonlyPaths
+                .clone_from(&c_settings.Linux.ReadonlyPaths);
         }
 
         let sandbox_pidns = if is_pause_container {
@@ -722,7 +724,7 @@ fn get_image_layer_storages(
             "previous_chain_id = {}, chain_id = {}",
             &previous_chain_id, &chain_id
         );
-        previous_chain_id = chain_id.clone();
+        previous_chain_id.clone_from(&chain_id);
 
         layer_names.push(name_to_hash(&chain_id));
         layer_hashes.push(layer.verity_hash.to_string());

src/tools/genpolicy/src/registry.rs

@@ -215,7 +215,7 @@ impl Container {
 
         if let Some(working_dir) = &docker_config.WorkingDir {
             if !working_dir.is_empty() {
-                process.Cwd = working_dir.clone();
+                process.Cwd.clone_from(working_dir);
             }
         }
 
@@ -344,6 +344,7 @@ pub fn add_verity_to_store(cache_file: &str, diff_id: &str, verity_hash: &str) -
         .read(true)
         .write(true)
         .create(true)
+        .truncate(false)
         .open(cache_file)?;
 
     let mut data: Vec<ImageLayer> = if let Ok(vec) = serde_json::from_reader(read_file) {