ci: inherit --jobs via MAKEFLAGS in run-build-and-tests

Let's not decide in the generic ci/ script how many jobs to run in
parallel; it is easy enough to hand that information down via the
`MAKEFLAGS`.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

ci/run-build-and-tests.sh

@@ -7,7 +7,7 @@
 
 ln -s "$cache_dir/.prove" t/.prove
 
-make --jobs=2
+make
 make --quiet test
 if test "$jobname" = "linux-gcc"
 then