[CI] Cancel builds on subsequent pushes (#6011)

* [CI] Cancel builds on subsequent pushes

* Use a more secure method

* test commit

Jenkinsfile

@@ -38,12 +38,15 @@ pipeline {
       agent { label 'job_initializer' }
       steps {
         script {
+          def buildNumber = env.BUILD_NUMBER as int
+          if (buildNumber > 1) milestone(buildNumber - 1)
+          milestone(buildNumber)
+
           checkoutSrcs()
           commit_id = "${GIT_COMMIT}"
         }
         sh 'python3 tests/jenkins_get_approval.py'
         stash name: 'srcs'
-        milestone ordinal: 1
       }
     }
     stage('Jenkins Linux: Formatting Check') {
@@ -57,7 +60,6 @@ pipeline {
             'doxygen': { Doxygen() }
           ])
         }
-        milestone ordinal: 2
       }
     }
     stage('Jenkins Linux: Build') {
@@ -80,7 +82,6 @@ pipeline {
             'build-jvm-doc': { BuildJVMDoc() }
           ])
         }
-        milestone ordinal: 3
       }
     }
     stage('Jenkins Linux: Test') {
@@ -103,7 +104,6 @@ pipeline {
             'test-r-3.5.3': { TestR(use_r35: true) }
           ])
         }
-        milestone ordinal: 4
       }
     }
     stage('Jenkins Linux: Deploy') {
@@ -114,7 +114,6 @@ pipeline {
             'deploy-jvm-packages': { DeployJVMPackages(spark_version: '3.0.0') }
           ])
         }
-        milestone ordinal: 5
       }
     }
   }