Exposing global "REPLIT_LD_LOADER" property to be picked up by replenv builder

[blast-hardcheese]

Why

For better or worse, uv mandates the virtualenv being a "real" virtualenv, including bin/python symlinks. This subverts our attempts to manipulate the environment via shell-script wrappers, causing runtime library issues.

By moving the environment shuffling into the replenv builder itself, curating the environment itself instead of the wrapper scripts, we can have both uv as well as tools like rtld-loder.

What changed

• Expose REPLIT_LD_AUDIT

Test plan

I should be able to echo $REPLIT_LD_AUDIT and get the value defined even on an empty Repl.

Rollout

• This is fully backward and forward compatible

[blast-hardcheese]

I've confirmed that this puts the entry I need in the right place -- the intent is that this will replace not only the python wrappers but also the node/bun wrappers as well.

[airportyh]

Question, does this mean we are not setting this var at all for our current python setup?

[blast-hardcheese]

@airportyh Currently the value is interpolated into the Python wrapper directly, but I'm trying to move it to "globally available" if the flag is turned on.

Is it OK to be globally enabled for all binaries within a Repl? I would presume yes, at a possible slight impact to startup speed, but effectively a no-op after that, but we'd get the benefit of broken applications and libraries working out of the box without further patching.