fix: Add libdir to library search path

[shs96c]

We discovered when dealing with libraries such as psycopg2 that the wheel would attempt to link against libpython.a. This fix points the linker at the correct python version being used.

[aignas]

Because this is gated by a feature toggle, it seems to be low risk to include.

[rickeylev]

Just want to respond to something the comment says:

# [calling the python interpreter in repo-phase]
# is super hacky, but working out something nice is tricky.
# This is in particular needed for psycopg2 which attempts to link libpython.a,
# in order to point the linker at the correct python intepreter.

It's not possible to reliably get the correct Python interpreter. This is because the repo phase doesn't know, and can't know, what the build phase is going to do (i.e. what python interpreter will be used). The basic change in this PR is somewhat an improvement, but yeah -- just wanted to point out it'll still be prone to issues. To really fix this, the sdist building needs to move into the build phase.

[aignas]

I'll add some ideas about this in #2410.

[shs96c]

Is there anything else you'd like me to do for this PR?