Pass GNU Make jobserver on to the ci jobs

Solution found by reading the question [Is it possible to “pass-through”
GNU make jobserver environment to a submake served via a 3rd-party
(non-make)](https://stackoverflow.com/q/29910944/377022).

This, I hope, will fix errors such as
```
make[2]: *** write jobserver: Bad file descriptor.  Stop.
make[2]: *** Waiting for unfinished jobs....
make[2]: *** write jobserver: Bad file descriptor.  Stop.
make[1]: *** [coqprime] Error 2
make[1]: INTERNAL: Exiting with 1 jobserver tokens available; should be 2!
make[1]: Leaving directory `/home/travis/build/JasonGross/coq/_build_ci/fiat-c
```
which result from having a top-level `make` which sets up the jobserver
(via `-jN`), which invokes a non-makefile script *without passing on the
file descriptors for the jobserver*, which either invokes a makefile
script without `-jN` or invokes a makefile script with `-jN` which itself
invokes a submake without `-jN`.  This was the case, for example, in
fiat-crypto.

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