ci: Constrain parallel build jobs

The default `_NPROCESSORS_ONLN` heuristic we have isn't cgroups aware.
So it thinks it has e.g. 40 CPUs when running in a k8s pod. This can
then blow through our allocated resource limits.

Declare some modest amount of RAM and CPU resources and override `make`
parallelism.

This matches what rpm-ostree now does in
https://github.com/coreos/rpm-ostree/pull/2155.

ci/libbuild.sh

@@ -10,7 +10,7 @@ pkg_upgrade() {
 }
 
 make() {
-    /usr/bin/make -j $(getconf _NPROCESSORS_ONLN) "$@"
+    /usr/bin/make -j ${MAKE_JOBS:-$(getconf _NPROCESSORS_ONLN)} "$@"
 }
 
 build() {