The -s flag assures that nothing can be installed to user site-packages
but doesn't keep ensurepip from looking there for Pip.
If Pip is installed in the user site-packages directory, pip won't be installed
for the newly built python -- and its shim won't be created.
-I makes the build install Pip in any case.
The user site-packages installation will override it --
but we'll have the shim at least.
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