[7.15](backport #28481) CI: disable e2e for elastic-agent on tags/branches

[mergify]

This is an automatic backport of pull request #28481 done by Mergify.
Cherry-pick of d1bcb50 has failed:

On branch mergify/bp/7.15/pr-28481
Your branch is up to date with 'origin/7.15'.

You are currently cherry-picking commit d1bcb503b7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Jenkinsfile
	both modified:   x-pack/elastic-agent/Jenkinsfile.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.io/

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/7.15/pr-28481 upstream/mergify/bp/7.15/pr-28481
git merge upstream/7.15
git push upstream mergify/bp/7.15/pr-28481

[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: 2021-10-18T13:44:31.856+0000

• Duration: 216 min 51 sec

• Commit: b070c8d

Test stats 🧪

Test	Results
Failed	0
Passed	53139
Skipped	5274
Total	58413

💚 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 and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.