ci(jenkins): revert none agent (#398)

* Revert "ci(jenkins): Fix environment variable used to calculate git commit (#393)"
This reverts commit a5f46c8.
* Revert "ci(jenkins): avoid using the any agent to skip running in the master-worker (#391)"
This reverts commit 6e71ea3.

Jenkinsfile

@@ -11,14 +11,8 @@ it is need as field to store the results of the tests.
 */
 @Field def rumTasksGen
 
-/**
-This is the git commit sha which it's required to be used in different stages.
-It does store the env GIT_BASE_COMMIT
-*/
-@Field def gitCommit
-
 pipeline {
-  agent none
+  agent { label 'linux && immutable' }
   environment {
     REPO = 'apm-agent-rum-js'
     BASE_DIR = "src/github.com/elastic/${env.REPO}"
@@ -66,9 +60,6 @@ pipeline {
             deleteDir()
             gitCheckout(basedir: "${BASE_DIR}", githubNotifyFirstTimeContributor: true)
             stash allowEmpty: true, name: 'source', useDefaultExcludes: false
-            script {
-              gitCommit = env.GIT_BASE_COMMIT
-            }
           }
         }
         /**
@@ -179,10 +170,10 @@ pipeline {
             log(level: 'INFO', text: 'Launching Async ITs')
             build(job: env.ITS_PIPELINE, propagate: false, wait: false,
                   parameters: [string(name: 'AGENT_INTEGRATION_TEST', value: 'RUM'),
-                               string(name: 'BUILD_OPTS', value: "--rum-agent-branch ${gitCommit}"),
+                               string(name: 'BUILD_OPTS', value: "--rum-agent-branch ${env.GIT_BASE_COMMIT}"),
                                string(name: 'GITHUB_CHECK_NAME', value: env.GITHUB_CHECK_ITS_NAME),
                                string(name: 'GITHUB_CHECK_REPO', value: env.REPO),
-                               string(name: 'GITHUB_CHECK_SHA1', value: gitCommit)])
+                               string(name: 'GITHUB_CHECK_SHA1', value: env.GIT_BASE_COMMIT)])
             githubNotify(context: "${env.GITHUB_CHECK_ITS_NAME}", description: "${env.GITHUB_CHECK_ITS_NAME} ...", status: 'PENDING', targetUrl: "${env.JENKINS_URL}search/?q=${env.ITS_PIPELINE.replaceAll('/','+')}")
           }
         }