MakeOfficialBuildId= the default for source-only non-official build setup

[ViktorHofer]

Same issue as #4613 but for source-only builds

This needs to wait until we snap for Preview 3 to only apply to Preview 4 and later which is when we start shipping from the VMR.

[MichaelSimons]

[Triage] Some additional changes are needed as a result of making this. The source build release process would need to include the OfficialBuildId info as part of the release. Additionally when doing the final builds, there needs to be a source build CI run with the same OfficialBuildId input in order to produce the artifacts and sdks that get uploaded to blob storage. How would these builds get queued is something that needs to be flushed out.

[MichaelSimons]

Stable versioning would make this UX a lot better for end users as they would not have to specify the OfficialBuildId.

[mmitche]

Notes from the discussion after the UB Sync:

Rough ship plan

Preview 4
.NET ships a VMR build with a single unified official build ID. We notify distro partners what the input official build ID is.

Preview 5
.NET and distro partner builds ship stable bits

Work items

• Verify that source-build works with OfficialBuildId being set by the AzDO pipeline
• Verify that stable preview builds work as expected.
• Decide how stable builds should get turned on when it comes time to ship GA or a preview.
• Add a source-build leg to the MSFT VMR build that produces the source-built artifacts.
• Add/move additional verification legs into new pipelines that follow on from the official build.

[mmitche]

#4912 dotnet/dotnet#1070 #4914 #4915 dotnet/dotnet#1069

[ViktorHofer]

I wonder which set of nuget packages scenario-tests will use when testing source-only locally (-dev) or in CI (-ci). There aren't matching Microsoft produced nuget packages in those configurations. Question for @mthalman and @MichaelSimons