Resolving 'guix shell' Permission Errors and Build Failures: Returning to Cached Package State After Incremental Updates

[outlook user <RACP@outlook.fr>]

`guix shell` complains about

> guix shell: erreur : open-file : Permission non accordée: "/gnu/store/64j9m88x4hicgrqb3447fv7ra0chdk3a-guix-module-union/share/guile/site/3.0/gnu/packages[...]"

For admin and acl until I `# chmod o+r /gnu/store/64j9m88x4hicgrqb3447fv7ra0chdk3a-guix-module-union/share/guile/site/3.0/gnu/packages[...]`

Then it complains about 

> \ phase « configure »builder for `/gnu/store/llm04gk92jf72j4kvcr608s6rwm5pf5z-shadow-4.15.1.drv' failed with exit code 1
> la compilation de /gnu/store/llm04gk92jf72j4kvcr608s6rwm5pf5z-shadow-4.15.1.drv a échoué

I don't really understand. I pulled a lot of things and then I re-did a pull, now I need to re-download a new version of everything. It's like:

Iteration 1: Stable working
Iteration 2: Re-downloaded everything
Iteration 3: Hasn't downloaded much and don't have bandwidth for, now

I want to return to iteration 2 because it's a big update from iteration 1, and I don't want iteration 3 because I don't have bandwidth anymore to re download everything a new time

But those iterations aren't `guix system reconfigure` because I didn't performed any since iteration 1. It's only about some `guix refresh --update` `guix package --upgrade` `guix update` `guix upgrade` `guix pull`. It's the result of those commands from which I want to return to "iteration 2", because I have cached a lot of packages that I need, but I performed those commands times more after having mass cached what I need. I want to return to that iteration that has a lot of local cached packages. And eventually resolve that problem about open-file and building shadow