ci: force GO_VERSION

[v1v]

What does this PR do?

Set the environment variable earlier

Why

withMageEnv calls withGoEnv and it does use goDefaultVersion but for some reason it does not use the .go-version file but the default hardcoded version in https://github.com/elastic/apm-pipeline-library/blob/54b4c9b43747d5a0c58bb8605f9f0c0d2d6510d5/vars/goDefaultVersion.groovy#L41

[v1v]

Interestingly, I could not reproduce the issue in my local environment

pipeline {
    agent any

    stages {
        stage('Build') {
            environment {
                BASE_DIR = 'src/github.com/foo'
            }
            steps {
                dir("${BASE_DIR}") {
                    git branch: 'main', url: 'https://github.com/elastic/elastic-agent.git'
                    sh "cat .go-version"
                }
                script {
                    def value = goDefaultVersion()
                    echo "${value}"
                }
            }
        }
    }
}

produced

[elasticmachine]

💚 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: 2022-09-14T15:39:58.015+0000

• Duration: 60 min 11 sec

Test stats 🧪

Test	Results
Failed	0
Passed	9849
Skipped	35
Total	9884

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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

• /test : Re-trigger the build.

• /package : Generate the packages.

• run integration tests : Run the Elastic Agent Integration tests.

• run end-to-end tests : Generate the packages and run the E2E Tests.

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

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	97.333% (73/75)	👍
Files	68.085% (160/235)	👍
Classes	68.478% (315/460)	👍
Methods	52.826% (888/1681)	👍
Lines	38.846% (9548/24579)	👎 -0.037
Conditionals	100.0% (0/0)	💚