chore(ci): be more specific about the underlying ARM OS (#1341) (#1344)

(cherry picked from commit 964b8a1)

Co-authored-by: Manuel de la Peña <mdelapenya@gmail.com>

.ci/Jenkinsfile

@@ -315,8 +315,8 @@ def generateFunctionalTestStep(Map args = [:]){
   def workerLabels = "${platform} && immutable"
   if (platform == "arm64") {
     goArch = "arm64"
-    // ARM is not ready as immutable workers
-    workerLabels = "arm"
+    // let's use centos8 ARM as base VM, trying to mitigate the flakiness described in https://github.com/elastic/e2e-testing/issues/1271
+    workerLabels = "centos-8-aarch64"
   }
 
   // Skip scenarios per platform