install command doesn't use platform in user site directory on Windows

[Steve Dower]

In sysconfig, the nt_user scheme includes a platform specifier to distinguish between 32-bit, 64-bit and ARM64 builds.

This is missing from the install command, which is used by setuptools (and indirectly, pip) to determine the location of the user site in some cases. As a result, on non-x64 platforms, the path is incorrect.