Helper step to get the current build time

[v1v]

What does this PR do?

Helper step to get the current build time with the format xs:datetime, for example 1980-03-23T10:20:15

Why is it important?

Avoid errors with the script approval

Related issues

Notifies elastic/beats#31355 elastic/elastic-package#792 elastic/integrations#3126

ITs

Given

@Library('apm@current') _

pipeline {
  agent any
  stages {
     stage('Echo') {
        steps {
           echo getCurrentBuildTime()
        }
     }
  }
}

produced

[apmmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-04-20T09:39:50.004+0000

• Duration: 8 min 11 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1420
Skipped	2
Total	1422

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run infra tests : Run the test-infra test.

• /test all : Re-trigger the build for all the stages.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)