Allows include build number for packages blocked from going stable

[weshaggard]

For any packages that we doin't want to ship and marked as
BlockStable we should always force include the label and
build number in the package name.

cc @mmitche @ericstj

This changes (3) and (4) from the original commit 4fcf6cf.

3. StabilizePackageVersions=true, PackageVersionStamp='label'

• Packages not marked BlockStable will have prelease label 'label'
  and no build number
• BlockStable packages will have a prelease label 'label' and build
  number

4. StabilizePackageVersions=true, PackageVersionStamp=''

• Packages not marked BlockStable will not have a prelease label
  or build number, and thus have a stable version.
• BlockStable packages will have the prelease label commited in the
  repo and build number

[weshaggard]

@dotnet-bot test this please

[weshaggard]

@dotnet-bot test NETFX x86 Release Build

[weshaggard]

netfx test run hit https://github.com/dotnet/corefx/issues/30713.