Add DebugType as portable, to generate deterministic nupkgs in 1ES

[davidmrdavid]

While migrating to our new 1ES CI, we realized that the nupkgs generated by this repo were not getting the "deterministic" flag in the NuGet package explorer.

After a lot of experimentation, we discovered the reason for this was that our packages did not set the DebugType MSBuild property to embedded portable which apparently is needed for deterministic builds. The need for this property (and others we were already setting) is explained here: https://www.meziantou.net/creating-reproducible-build-in-dotnet.htm

To be honest, I don't know why previous CI environment still generated deterministic builds, even without this flag. That's a mystery for another day. For now, and to unblock upcoming releases, we'll set the DebugType of our packages to embedded portable to all the expected checkmarks in the NuGet package explorer.

For comparison, here's the view of the NuGet package before this change:

and here it is after:

Edit 9/18: a previous version of this PR set the DebugType as embedded. I've set it to portable on this date based on this feedback: #343 (comment)

[cgillum]

I recall that Jacob had preferred non-embedded symbols in our nuget package, and instead wanted us to publish symbol packages separately (snupkg files). Unfortunately, I don't have experience working with snupkg files, but I suspect the DebugType value for this is Portable, based on the metadata I see in previous versions of these packages.

I don't think we need to block on this, but it might be worth taking a look to see whether this might impact the release process, if for example it expects to find snupkg files but none are generated (because we use embedded symbols).

[davidmrdavid]

I made a test on 1ES and seems portable works as well to get the expected checkmarks. I'll set it to portable based on this recollection