bug#73304: Python in relocatable guix pack leads to wrong sys.path

[Rutherther]

When I add both python and a python package (seems like any) to a
relocatable guix pack, the resulting python in the merged profile seems
to be broken. Specifically its `sys.path` contains missing paths.

To replicate:
```
mkdir -p /tmp/b && cd /tmp/b
tar xzvf $(guix pack -RR python python-pyflakes -S /profile=)`
```

Then try executing the python from merged profile, and see sys.path output
--8<---------------cut here---------------start------------->8---
./profile/bin/python3
>>> import sys
>>> print(sys.path)
['', 
'/tmp/b/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python310.zip', 
'/tmp/b/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python3.10', 
'/tmp/b/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python3.10/lib-dynload',
 
'/tmp/b/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python3.10/site-packages']
--8<---------------cut here---------------end--------------->8---

Now if you instead use the realpath of this python
--8<---------------cut here---------------start------------->8---
$(realpath ./profile/bin/python3)
>>> import sys
>>> print(sys.path)
['', 
'/tmp/b/gnu/store/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python310.zip',
 
'/tmp/b/gnu/store/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python3.10',
 
'/tmp/b/gnu/store/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python3.10/lib-dynload',
 
'/tmp/b/gnu/store/pwajwfiygrkmag8fm1wzn3cf362j7sri-python-3.10.7R/lib/python3.10/site-packages']
--8<---------------cut here---------------end--------------->8---

As can be observed, the first instance misses "gnu/store" in the path,
and instead points to a path that does not exist. I have not
investigated, at least not yet, where this discrepancy comes from.
I was not able to replicate this with anything else than relocatable
pack. So my guess would be something strange happens when making
bwrap/fakechroot etc.

Regards,
Rutherther