fix(pytest): support CARGO_BUILD_TARGET in conftest.py

We have to specify custom targets to disable ASan for build.rs and such.
This affects the path to picodata, so we have to adapt:

```
target/debug/picodata
target/x86_64-unknown-linux-gnu/asan/picodata
```

Also deduce target for ASan build from `rustc -vV`.

Co-Authored-By: Dmitry Rodionov <d.rodionov@picodata.io>
Co-Authored-By: Dmitry Ivanov <d.ivanov@picodata.io>