Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

add a .unpacked file in the env prefix folder to show the unpack status.

Why is this needed?

the unpack script my run concurrent, or do a meaningless unpack.

What should happen?

• concurrent run conda-unpack should be ok.
• we can detect whether conda-unpack has already run.

Additional Context

No response