ci: use the Jenkins Code Coverage Plug-in with source support

[v1v]

Trying to enrich the CI with the source integration as stated in #585 (comment)

Test

If you go to here

Then you will see the code coverage.

There are some files that are not found, so no idea about the intrinsic behaviour for the codecoverage either for this project or in the CI.

Logs can be found here

Or see the below extract from the entire log output while using the new coverage step in the CI.

[2022-04-01T14:00:41.334Z] Copied cmd/promote.go. 
[2022-04-01T14:00:41.334Z] Starting copy source file internal/testrunner/testrunner.go. 
[2022-04-01T14:00:41.354Z] Copied internal/testrunner/testrunner.go. 
[2022-04-01T14:00:41.354Z] Starting copy source file internal/profile/profile.go. 
[2022-04-01T14:00:41.369Z] Copied internal/profile/profile.go. 
[2022-04-01T14:00:41.369Z] Starting copy source file multiinput/test. 
[2022-04-01T14:00:41.381Z] Unable to find source file multiinput/test in workspace /var/lib/jenkins/workspace/t-manager_elastic-package_PR-775/src/github.com/elastic/elastic-package
[2022-04-01T14:00:41.381Z] Starting copy source file apache/status. 
[2022-04-01T14:00:41.392Z] Unable to find source file apache/status in workspace /var/lib/jenkins/workspace/t-manager_elastic-package_PR-775/src/github.com/elastic/elastic-package

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2022-04-05T08:44:10.951+0000

• Duration: 32 min 50 sec

Test stats 🧪

Test	Results
Failed	0
Passed	628
Skipped	0
Total	628

🤖 GitHub comments

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

• /test : Re-trigger the build.

[mtojek]

/test

[v1v]

I'll close this PR since https://github.com/elastic/apm-pipeline-library/releases/tag/v1.1.313 is now supporting this feature implicitly. The coverageReport step is the one now driving this