pyoxidizer: force pip to not use pep517 in order to be able to install hg
Mercurial is not (yet) a pep517 package, but the presence of a pyproject.toml
file tells newer-ish versions of pip that it should be one.
This is related to 58fe6d127a01, and fixes pyoxidizer builds for the Heptapod
Differential Revision: https://phab.mercurial-scm.org/D11710