Revert build dependency installer isolation for legacy build environment (#14228)

This was unneeded for the legacy isolation method (as --ignore-installed
is used) and stops pip from being able to use the same keyring available
in a venv while installing build dependencies (note that the virtual
environment isn't activated so the subprocess keyring provider is useless
here).

This is harder to fix (and thus left unfixed) for venv-isolation since
the subprocess installer will use the temporary venv's Python executable
to run pip, so there's no way for said pip subprocess to directly import
keyring. The proper fix is to switch to the inprocess installer when using
venv isolation.

So, I guess the real answer to everything is fix inprocess-build-deps
and venv-isolation compatibility and aim to enable both together 🙃.