Disable lockfile under fleet mode

[fearful-symmetry]

What does this PR do?

This is the other half of the fix for #31670 (comment)

This disables the lockfile when the beat is running under elastic-agent.

I've tested this, but I'm not 100% sure on what, if any, unintentional behaviors this might cause. Theoretically elastic-agent should be responsible for correctly starting/restarting a beat, and elastic-agent has its own lockfile.

Why is it important?

We're having a lot of issues with lockfiles under elastic-agent.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @fearful-symmetry? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2022-12-12T20:15:23.659+0000

• Duration: 69 min 58 sec

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Test	Results
Failed	0
Passed	25089
Skipped	1954
Total	27043

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[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 lockfile-fleetmode-disable upstream/lockfile-fleetmode-disable
git merge upstream/main
git push upstream lockfile-fleetmode-disable

[cmacknz]

This change looks straightforward enough. Likely we have to let this marinate on main for a while to see if anything surprising happens.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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