Stops adding the IOx source code and performing a cargo build/test/clippy each
night. Previously this build would compile the IOx source & dependencies,
populating the incremental build cache and allowing builds that used the same
dependencies to complete quicker. This build caching was moved to
per-dependency-set caching in #496, and this pre-build is no longer used.
This should reduce the build image size substantially, making the whole CI
process a bit faster.
Dom
Andrew Lamb