Fix nuget config override option, set default warp version to 1.0.15

[bob80905]

This PR fixes the current failures in the PR pipelines by defaulting the nuget warp install version to 1.0.15 for the pipelins. Devs will still pull the latest warp version.
It also fixes the override option that wasn't quite working before, so that the desired nuget warp version can be overridden.
When invoking hctbuild, just pass -nuget-config "-Version " to install a specific version of warp.

[damyanp]

LGTM assuming build succeeds.

It'd be nice if the default was set in DownloadWarp.cmake so that the knowledge about which version of warp to use wasn't baked in to azure-pipelines.yml, but we can always make that change later.

[damyanp]

actually it looks like the new good version is 1.0.16.1

[damyanp]

Looks good for what it's doing here, but I'd prefer to see the LKG hard coded in hctbuild.cmd or DownloadWarp.cmake so we don't have to put it into every pipeline yml file.

Happy to leave changing that for another day.

[bob80905]

Looks good for what it's doing here, but I'd prefer to see the LKG hard coded in hctbuild.cmd or DownloadWarp.cmake so we don't have to put it into every pipeline yml file.

Happy to leave changing that for another day.

Jesse mentioned he published the last Warp version specifically for the purpose of getting bug reports from users. So there is some advantage in leaving hctbuild / DownloadWarp.cmake to download the latest warp, so that devs can file reports. Devs can override the warp version if they need to be unblocked too.
Secondly, I think it's good to place the responsibility on hctbuild to use the latest WARP. Something should use the latest. If not, we may just entirely forget to update to newer warp versions. As new versions are released, and if things break, devs will see it, and will know whether to update the LKG in the pipelines or not.