Contributes to #3739
Depends on #4215

A join point vertical job isn't much different to a normal vertical vmr-build job aside from:

Two new parameters

1. reuseBuildArtifactsFrom: This parameter lists the jobs that the join point job depends on. This parameter should support multiple values and the order is important. Artifacts from the job name listed first are preferred in case of duplicates with other jobs listed here.
2. buildPhase: This parameter identifies the build phase of the join point job. This should be a value > 1.

The existing job steps need to be updated to also work in a non-source-only build mode: https://github.com/dotnet/installer/blob/986cd8e87e84681dfa2dc11efa85ce2a2b15176e/eng/pipelines/templates/jobs/vmr-build.yml#L168-L210

The artifacts from the previously built verticals should be copied into the VMR's root artifacts folder. If a file already exists then it should not overwritten. This enables the priority system so that i.e. win-x64 artifacts are preferred over win-x86.