fix(toolchain): symlink all toolchain files for the host toolchain (#1745)

Previously we would only symlink the interpreter binary itself
when creating the hermetic host toolchain used in setting up the
`whl_library` repositories. This works on UNIX platforms and Windows
if they have the following in their `.bazelrc`:
```
startup --windows_enable_symlinks
```

In our CI we had the same lines but the users did not need to add them
until the `0.29.0` forced them to have them because we actually started
using symlinks on Windows. If the symlinks are not enabled on the host
platform `bazel` tries to be helpful and copies the files over instead
of making the links. We are leveraging this to just symlink all of the
contents of the python interpreter repository for the host platform to
the `_host` toolchain repository.

Fixes #1723