For 5.0.0, use bootstrapping flow for previously-source-built nupkgs rather than downloading them from N-1 #1880

• Could stop using the cached N-1 => N flow. (Until we get into 5.0 servicing?)

I brought this up with the team and we think it makes sense for 5.0.0. Instead of downloading a previously-source-built.tar.gz, we'll pack up the nupkgs produced by the production build. I'm hopeful it's a tweak to the tarball generation code rather than something drastic. (We'll still produce previously-source-built.tar.gz files so that we can go back to a sort-of N-1 => N flow for 5.0.1.)

#1828 (comment)

With this, we shouldn't have to do #1878, because they solve the same problem.