[MacSDK] Port two msbuild fixes from private bockbuild

[MacSDK] Port two msbuild fixes from private bockbuild

https://github.com/xamarin/bockbuild/commit/58467b852e13b4bdfc75a2a0f9b9507b417b0694:

    [msbuild] Create symlinks from lib/mono/xbuild/Microsoft/NuGet/*
    msbuild install script cannot do this as the source files are installed
    by mono, which are in a different staging directory.

https://github.com/xamarin/bockbuild/commit/fb9656a7613bab2d77c8ce7e05d673c5abbbdac5:

    Partially Revert "[msbuild] Remove redundant msbuild logic from mono_master.py"

    Use 15.0 instead of mono version! msbuild's install script cannot
    copy these since bockbuild installs msbuild to a separate staging
    directory from mono. So, it needs to be done here.