Add CODE_COVERAGE option to enable coverage instrumentation in builds.
Add new CI job to upload coverage reports to CodeCov.io.
Reports can be found by navigating the codecov.io UI, or by going to
the URL printed at the end of the "Code Coverage" CI job. More CodeCov
integration could be set up later once we have coverage reporting on the
main branch.
This reduces the time to run tests from 5-9 minutes down to 3-4 minutes.
`nproc` is returning `1` on the CI node which is processing our
config.yml. In the absense of a better way of getting the number of CPUs
on our executor, just hardcode it.
We actually have 16 cores on our 2xlarge executors, but higher
parallelism in tests appears to cause oid to sometimes fail with SIGILL.
I'm still investigating this problem.
This will give us more flexibility later, e.g. allowing different steps
for GCC and Clang (as needed for code coverage), or letting us
parallelise our tests across multiple machines.
