[Linux] Packages cannot be installed into `/`

When using the glibc standalone Python builds for Linux containers, installing these standalone builds into `/` results in some weird behaviour. The root cause appears to be that `sys.base_prefix` is set to `/install`. I've created a Rockcraft file that demonstrates the issue at https://github.com/lengau/bug-reproducers/tree/python-standalone-bin

While the packages work correctly when placed into `/usr`, they don't work in `/`. If placed in `/usr` on a system with [/usr merge](https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/), the same issues appear when calling `/bin/python3` or related.

Given how different this is from the purpose of these builds I think it's reasonable to file it under quirks and move on, but I figured you ought to know :-)

Start with the linked Rockcraft reproducer and compare the standalone build installed into `/` with the same build under `/usr`. Check the reported `sys.base_prefix` value and the behavior of `/bin/python3` on a usr-merged system. Done would require a confirmed explanation and a decided fix or documented quirk, but the issue does not name repository files or tests.

Written by the indexing model from the issue text.