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

contrib/install-windows-dependencies.ps1

@@ -147,7 +147,7 b' function Install-Dependencies($prefix) {'
     Install-Python3 "Python 3.7 32-bit" ${prefix}\assets\python37-x86.exe ${prefix}\python37-x86 ${pip}
     Install-Python3 "Python 3.7 64-bit" ${prefix}\assets\python37-x64.exe ${prefix}\python37-x64 ${pip}
     Install-Python3 "Python 3.8 32-bit" ${prefix}\assets\python38-x86.exe ${prefix}\python38-x86 ${pip}
-    Install-Python3 "Python 3.8 64-bit" ${prefix}\assets\python38-x64.exe ${prefix}\python38-x64 ${pip}
+#    Install-Python3 "Python 3.8 64-bit" ${prefix}\assets\python38-x64.exe ${prefix}\python38-x64 ${pip}
     Install-Python3 "Python 3.9 32-bit" ${prefix}\assets\python39-x86.exe ${prefix}\python39-x86 ${pip}
     Install-Python3 "Python 3.9 64-bit" ${prefix}\assets\python39-x64.exe ${prefix}\python39-x64 ${pip}