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 ostree does in
https://github.com/ostreedev/ostree/pull/2151.

.cci.jenkinsfile

@@ -40,9 +40,10 @@ codestyle: {
   }
 },
 rust: {
-  cosaPod(buildroot: true, runAsUser: 0, memory: "2Gi") {
+  def n = 5
+  cosaPod(buildroot: true, runAsUser: 0, memory: "2Gi", cpu: "${n}") {
       checkout scm
-      shwrap("ci/msrv.sh")
+      shwrap("MAKE_JOBS=${n} CARGO_BUILD_JOBS=${n} ci/msrv.sh")
   }
 }}
 

ci/libbuild.sh

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