Update pipeline to specify `--packages` option for previously source built scenarios

[mthalman]

The VMR build script currently allows for previously source built packages to be provided implicitly, by storing them in the prereqs/packages/archive directory, or via an explicit path set with the --packages option. The latter is not exercised by any automation which led to a regression that went unknown for a while.

The validation pipeline does exercise the scenario of using previously source built packages, but this is done by explicitly copying the packages to the well-known prereqs/packages/archive location. This avoids the need to specify the --packages option to the build script. The problem with that is there is no test coverage of the --packages option.

The proposal here is to update the pipeline so that the previously source built scenarios will explicitly set the --packages option to a location other than prereqs/packages/archive.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ViktorHofer]

Agreed. Does anyone (devs) use the well-known location today? If not, I would prefer to remove that functionality and require bits to be passed-in explicitly via the --with-packages option. Related: #4422

[mthalman]

Yes, it's used by the prep script which is a very common dev scenario. Having to explicitly provide --with-packages to the build script in that scenario would be very annoying.