pwndbg: fix runtime python deps, fixes #71071

pwndbg is a Python module for gdb. The built-in interpreter is used and
pwndbg offers additional routines. Packaging this is tricky because that
interpreter needs to be used. Using `python3.withPackages` won't work.
By setting `NIX_PYTHONPATH`, the interpreter should pick up pwndbg and
its dependencies.

If `NIX_PYTHONPATH` does not function we can fall back to `PYTHONPATH`.
An example of when that won't work is if pwndbg runs a script of itself
in a subshell. `NIX_PYTHONPATH` would be  unset, but `PYTHONPATH` not.