add libabigail ABI checks to CI

[grondo]

It might be good to add ABI checks to CI to warn if and when PRs break ABI.

See https://github.com/buildsi/libabigail-action for an example of how this might be done.

Since we publish docker images of the latest merged changes, we might be able to do this without the parallel builds for comparison (just fetch the latest docker image and pull libflux-core.so from there).