ci: throttle builds to avoid tests clashing (#3127)

Uses:
https://plugins.jenkins.io/throttle-concurrents/#example-1-throttling-of-node-runs

The `nimbus-eth` category limits builds to one per node in global settings.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

Jenkinsfile

@@ -93,17 +93,21 @@ def runStages() {
 
 parallel(
 	"Linux": {
+		throttle(['nimbus-eth2']) {
 			node("linux") {
 				withEnv(["NPROC=${sh(returnStdout: true, script: 'nproc').trim()}"]) {
 					runStages()
 				}
 			}
+		}
 	},
 	"macOS": {
+		throttle(['nimbus-eth2']) {
 			node("macos") {
 				withEnv(["NPROC=${sh(returnStdout: true, script: 'sysctl -n hw.logicalcpu').trim()}"]) {
 					runStages()
 				}
 			}
+		}
 	},
 )