ci: Re-add CARGO_BUILD_JOBS

We lost this at some point during the CI re-shuffle. We need to
constrain cargo builds too to respect our CPU allocation.

This doesn't totally keep all jobs under 5 since e.g. we could have 5
make jobs and 5 cargo codegen builds going at once, but I think as long
as it's not something ridiculous like 40, it should be fine. Otherwise
we'll tighten it more.

.cci.jenkinsfile

@@ -5,7 +5,8 @@ parallel rpms: {
   def n = 5
   cosaPod(buildroot: true, runAsUser: 0, memory: "2Gi", cpu: "${n}") {
       checkout scm
-      shwrap("""RPM_BUILD_NCPUS=${n} ./ci/coreosci-rpmbuild.sh
+      // 2:1 job to CPU at most should keep us from getting kicked out
+      shwrap("""RPM_BUILD_NCPUS=${n} CARGO_BUILD_JOBS=${n} ./ci/coreosci-rpmbuild.sh
                 mv packaging/*/*.rpm .
              """)
       // make it easy for anyone to download the RPMs