[shell] move nix-profile into a specific directory inside .devbox directory

[savil]

Summary

To avoid the nix-profile symlinks crowding the .devbox/ directory, I push them
inside a .devbox/nix/profile/ sub-directory. The symlink is renamed to default inside .devbox/nix/profile to be consistent with nix naming.

I also want to rename profileDir to profilePath. That is more accurate since
it is not a directory but a symlink. However, will do in a separate PR since
it has some conflicts with concurrent work like #207.

How was it tested?

> cd testdata/rust/rust-stable
> devbox shell
> which rustc
# see path in nix store

> devbox add openssl
> hash -r
> which openssl
# see path in nix store

# inspect directory and symlinks:
> ls -al /Users/savil/code/jetpack/devbox/testdata/rust/rust-stable/.devbox/nix/profile/
total 0
drwxr-xr-x 5 savil staff 160 Oct  6 11:55 .
drwxr-xr-x 3 savil staff  96 Oct  6 11:55 ..
lrwxr-xr-x 1 savil staff  14 Oct  6 11:55 default -> default-2-link
lrwxr-xr-x 1 savil staff  60 Oct  6 11:55 default-1-link -> /nix/store/qn9px7zzx500d0l78nhz0b39v8nm1siq-user-environment
lrwxr-xr-x 1 savil staff  60 Oct  6 11:55 default-2-link -> /nix/store/6q654ka2lcdvamxxvnsa9vp9dcj9b6yg-user-environment

[savil]

Current dependencies on/for this PR:

• main
  • PR [shell] move nix-profile into a specific directory inside .devbox directory #210 👈

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[LucilleH]

move it under /gen?

[savil]

I initially felt we should reserve gen for generated files....maybe we should discuss the internal structure of .devbox folder.

[savil]

test failure is regression from #218. Tests pass locally if I delete testdata/python/pipenv.