ndi: retrieve tarball when users have expressed consent via $NIXPKGS_CONFIG

[peti]

The ndi build should automatically fetch the source tarball from upstream if (and only if) the user has configured some explicit setting in $NIXPKGS_CONFIG, confirming that they accept NewTek's EULA:

$ wget -O InstallNDISDK_v4_Linux.tar.gz http://new.tk/NDISDKLINUX

Other proprietary packages in Nixpkgs are doing the same thing, so it's probably okay. This feels simpler that forcing the user to download that file themselves.

[stale]

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