This is a follow-up to commit
fdc0fefd46
build-system/pyproject: Use python-sans-pip-wrapper as default-python.
* tests/guix-shell-export-manifest.sh: Grep for python-sans-pip-wrapper, which
is the python now used by the pyproject-build-system.
Change-Id: I5f44e07544f80973fa6b306b2dcdf42ac3b56ad6