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

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.