build: limit --all to 10 apps at a time

This needed now because the buildserver is hanging so often, that we are often going a week or more without any builds published. Perhaps this is only temporary, or maybe we will want to flush this feature out more as a standard thing. But we really need it for now to at least get some builds out on a daily basis.

Since the website deploy is also triggered by this cycle, making the build finish more often means the website will be published more often.

Given the recent history, limiting the build to 10 apps would still have hung about half the time. There were runs that froze after 1, 2, 5, and 7 apps.