Enable stage 2 and other non-MSFT builds in `main`

[NikolaMilosavljevic]

All non-MSFT (including stage 2) builds were disabled with dotnet/sdk#43439

We've disabled them to resolve #4586

Stage 2 builds were failing as is always the case during transition to a new release. The failures were originally introduced with dotnet/sdk#42969. We could have modified those properties, to enable PVP version flow, and get stage 2 builds running, but that would have only worked for a short time, until runtime and aspnetcore moved to 10.0 versioning. At that point building 9.0 projects could break, as _NET90RuntimePackVersion and other 9.0 'packs' properties in src/Installer/redist-installer/targets/GenerateBundledVersions.targets would reference 10.0 'packs'.

Other non-MSFT builds were disabled even though they were passing at the moment - however, the very next re-bootstrapping in main would have broken those.

All non-MSFT builds can be enabled after 9.0 GA, at which point we need to add 9.0 targeting pack to SBRP and update the properties introduced in dotnet/sdk#42969 to reference it.