two-pass build of 'system/' repository unnecessarily inefficient when certain packages fail to build

As currently designed, autobuilder performs two passes of the 'system/' repository. In both passes, the build order is determined by a topological sort based on dependencies. This part is fine.

Once 'system/' is built for all scheduled architectures, twice, this is no longer an issue.

If a package is rebuilt in the first pass, it may trigger a catastrophic chain reaction of builds. This can be prohibitively expensive on lower-powered build machines, or devastatingly annoying at best.

Implement a mechanism to protect against unnecessary rebuilds.