conda-pack should clean the prefix path in conda-meta/{pixi,pixi_env_prefix} files

[gzm55]

Checklist

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What is the idea?

when packing a pixi generated conda env folder, conda-meta/pixi and conda-meta/pixi_env_prefix should be also cleaned like other files under conda-meta/.

For conda-meta/pixi, the content is a json, the "manifest_path" can be set to empty string.
for conda-meta/pixi_env_prefix, the contenst is just the plain text of absolute path of conda-meta/ folder. It can be overwrited to a place holder when not specify a dest prefix.

Why is this needed?

No response

What should happen?

No response

Additional Context

No response