On the CentOS 7 build, test a make after a make clean. Should
catch the case where bundled generated headers files get deleted
when cbindgen is not available to rebuild them.
If sources are vendored, we get the same effect of using frozen
with a lock file, and the Cargo.lock is generated based
on the vendored sources.
This also removes the need to ship a Cargo.lock.
Fixed out of source builds with vendored sources.
As the action runs natively on 18.04 we were not explicitly
setting a container, but this means we're using what GitHub
provides us as a default state which might be broken. Instead
use the standard Ubuntu 18.04 container.