Autodetecting toolchain causes call cycle

[mattem]

When using the builtin autodetecting toolchain, it fails to correctly find the users python binary as it calls which python at runtime, at which point the venv has been activated and the wrapper script from the toolchain finds itself as the result of which python, therefore getting stuck in a loop.

We'd need either need to stipulate that this ruleset is incompatible with the builtin autodetecting toolchain or provide a better autodetecting toolchain that does the lookup at analysis time in a repo rule and therefore can be marked as configurable.

[mattem]

We'll add a new autodetecting toolchain here (or maybe rules_python?) and document the issue.