[Bug]: building chapel-py-venv removes sphinx-build binary

[mppf]

I'm observing a problem where make chplcheck-venv removes parts of the chpl-venv used for chpldoc.

$ make docs
$ ls third-party/chpl-venv/install/chpldeps/bin/
   # output includes sphinx-build
$ make chplcheck
$ ls third-party/chpl-venv/install/chpldeps/bin/
   # no sphinx-build

Here is a shorter way to reproduce it:

$ rm -Rf third-party/chpl-venv/{build,install}
$ make third-party-chpldoc-venv
$ ls third-party/chpl-venv/install/chpldeps/bin/
  # output includes sphinx-build
$ make chapel-py-venv
$ ls third-party/chpl-venv/install/chpldeps/bin/
  # no sphinx-build

A side issue is that make chapel-py-venv interleaved with make seems to always rebuild libChplFrontendShared.so.

It is unclear to me if this behavior has changed recently or if I'm just running into it now.