contrib: comment out the 64-bit py38 dependency installation on Windows
Not sure what is going on here, but it appears to not install py3.8 x64 in the
usual `C:\hgdev` directory. The x32 installer works fine. I'm assuming this is
a bug in this version of the installer, but didn't look into it too much.
Differential Revision: https://phab.mercurial-scm.org/D11274
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