Add option to not write out dependency graph spec

[jeffkl]

NuGet Product(s) Involved

MSBuild.exe, dotnet.exe

The Elevator Pitch

After restore, NuGet writes out a dependency graph spec (dgspec) in the form of JSON. This file is only used in the scenario where you have loaded projects in a Visual Studio solution but a project that isn't loaded is referenced. In this case, NuGet loads the dgspec file to get information for restore.

However, in hosted build environments like Azure DevOps, there is no need to write this file since it isn't used. In a very large repository I tested out not writing this file and it reduced restore time by almost 20%.

We would not want to have this option on by default and instead would encourage people to set something based on their environment:

<PropertyGroup>
  <!-- Do not write out NuGet's dgspec file in our hosted build environment -->
  <RestoreWriteDependencyGraphSepc Condition="'$(ContinuousIntegrationBuild)' == 'true'">false</RestoreWriteDependencyGraphSepc>
</PropertyGroup>

Additional Context and Details

Relevant code is at https://github.com/NuGet/NuGet.Client/blob/72ab0e05562bc8e19db1ce0c2630a80e0bd3ca28/src/NuGet.Core/NuGet.Commands/RestoreCommand/RestoreResult.cs#L205C19-L209