unpack to another writable dir

[gzm55]

Checklist

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

What is the idea?

when the packed .tgz is extracted into a read-only folder, we cannot run unpack script to resolve prefix path. The idea is that conda-unpack script could do a on-the-fly unpack action when copy/link the content of the read-only fold to another user specified writable target folder.

Why is this needed?

unpack on read-only folder is not possible, and copying full dir is expensive.

What should happen?

link the files to the target dir, and do unpack if needed.

Additional Context

No response