Specify python interpreter when installing pip dependencies for CI

[krishnangovindraj]

Usage and product changes

We now specify the python interpreter when installing pip dependencies for CI. This allows us to ensure the dependencies are installed for the specified toolchain.

Implementation

Introduce a python_interpreter_symlink repository which resolves toolchain based on information from repository_context.os. The symlink for the right version can then be passed to pip_parse as the python_interpreter_target argument.