fix: update mount dir for opam

Author: BoYanZh

jd4/_sandbox.pyx

@@ -67,6 +67,7 @@ def enter_namespace(root_dir, in_dir, out_dir):
     bind_mount(in_dir, 'in', True, False, True, False)
     bind_mount(out_dir, 'out', True, False, True, False)
     bind_mount('/root/.matlab', '.matlab', True, False, True, True, True)
+    bind_mount('/root/.opam', '.opam', True, False, True, True, True)
     write_text_file('etc/passwd', 'icebox:x:1000:1000:icebox:/:/bin/bash\n')
     mkdir('old_root')
     pivot_root('.', 'old_root')