backup: Set O_NOATIME in the right place

The archiver uses FS.OpenFile, where FS is an instance of the FS
interface. This is different from fs.OpenFile, which uses the OpenFile
method provided by the fs package.

internal/fs/fs_local.go

@@ -24,6 +24,7 @@ func (fs Local) Open(name string) (File, error) {
 	if err != nil {
 		return nil, err
 	}
+	_ = setFlags(f)
 	return f, nil
 }
 
@@ -37,6 +38,7 @@ func (fs Local) OpenFile(name string, flag int, perm os.FileMode) (File, error)
 	if err != nil {
 		return nil, err
 	}
+	_ = setFlags(f)
 	return f, nil
 }