Fix package restore in init.ps1

[AArnott]

When restoring packages for a project.json directly, as init.ps1 will do for nuproj projects, the config file is not automatically found by nuget.exe. This forces it to consider that, which lets the package restore succeed since the packages come from sources other than nuget.org.

[msftclas]

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[IlyaBiryukov]

Looks good

[AArnott]

Thanks.