Is mkdir-p a directory outside of /gnu/store.... possible?

[James Smith]

Hi,
I am using the guix package manager on Debian.
I guix package --install-from-file-myapp.scm and during the install process I 
would like to

(mkdir-p "$HOME/.config/myapp") and then
(system* (cp "/gnu/store/.../myapp/config.txt" "$HOME/.config/myapp"))

I launch guix package -i... from my home directory /home/jbs

I have tried

(mkdir-p "/home/jbs/.config/myapp")
(system* "mkdir" "-p" "/home/jbs/.config/myapp")
(invoke "mkdir" "-p" "/home/jbs/.config/myapp")

but get permissions errors. I can't even make the directory in /tmp...

I see the discussions on $HOME, homeless-shelter and the hacks in 
.../scripts/environment.scm so I presume I cannot determine $HOME(?) but even 
hardcoding my home does not work.

Is it possible during the make/install process to make a directory outside of 
/gnu/store... and if yes how?
Thanks
Jim