It has been proposed that make running too many parallel jobs recently
has led to resource exhaustion in our CI, so that some kind of limit
would be helpful to set.
The load average 2.5 limit choosen here is simply the limit used as an
example in the make documentation, as we don't really have any way of
picking an initial value that's certain to be better.
https://www.gnu.org/software/make/manual/html_node/Parallel.html
See #2421
See #2422
Remove the valgrind build in azure pipeline which is slow (about 50-75
mins to run) and replace with a set in github action that run 17
testing jobs in parallel to speed it up to about 25 mins the longest.
