sync: switch reader back to a shared lock

the below commit accidentally switched this lock to an exclusive lock
when it should just be a shared one as that is sufficient for a
reader:

e2c1866b: datastore/api/backup: prepare for fix of #3935 by adding
lock helpers

this has already caused failed backups for a user with a sync job that
runs while they are trying to create a new backup.

https://forum.proxmox.com/threads/165038

Signed-off-by: Shannon Sterz <s.sterz@proxmox.com>

src/server/sync.rs

@@ -480,7 +480,7 @@ impl SyncSource for LocalSource {
     ) -> Result<Arc<dyn SyncSourceReader>, Error> {
         let dir = self.store.backup_dir(ns.clone(), dir.clone())?;
         let guard = dir
-            .lock()
+            .lock_shared()
             .with_context(|| format!("while reading snapshot '{dir:?}' for a sync job"))?;
         Ok(Arc::new(LocalSourceReader {
             _dir_lock: Arc::new(Mutex::new(guard)),