Fix upgrade under V2

[belimawr]

What does this PR do?

Fix a typo that was preventing upgrades under V2.

Why is it important?

It fixes the upgrade process for versions >= 8.6.0

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 made corresponding change to the default configuration files
  - [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

## Author's Checklist

How to test this PR locally

1. Build the 8.6 branch (it will build the version 8.6.0)
2. Build the main branch (it will build the version 8.7.0)
3. Get a Elastic-Stack up and running with Fleet server
4. Install the Elastic-Agent v8.6.0
5. Ensure it is healthy
6. Stop the Elastic-Agent
7. Edit it's elastic-agent.yml and add the dropPath settings:

   agent.download:
     dropPath: /home/vagrant/dropPath

8. Restart the Elastic-Agent
9. Run the upgrade command as root (just to be on the safe side, also export the dropPath env var):

   AGENT_DROP_PATH=/home/vagrant/dropPath/ ./elastic-agent upgrade  8.7.0  -v -e -d "*"
   

10. Ensure the new version (v8.7.0) is installed by running elastic-agent status

Caveats

• On my tests after the upgrade the Elastic-Agent was unhealthy, that's probably due to the older Elastic-Stack.
• I'm currently working on unittests for the Elastic-Agent upgrade (Add integration and unit test coverage for the Elastic agent upgrade process. #1359, WIP test, it's not ready to be merged yet, but it shows that my fix works)

## Related issues
## Use cases
## Screenshots
## Logs

[mergify]

This pull request does not have a backport label. Could you fix it @belimawr? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

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

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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Build stats

• Start Time: 2022-11-24T09:15:38.507+0000

• Duration: 18 min 13 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4517
Skipped	13
Total	4530

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.305% (58/59)	👍
Files	71.066% (140/197)	👍
Classes	70.557% (266/377)	👍
Methods	54.828% (795/1450)	👍
Lines	40.341% (8595/21306)	👍
Conditionals	100.0% (0/0)	💚

[cmacknz]

Not even a one line fix, but one character fix. It could not get simpler.

Thanks for catching this and averting a future disaster.