Cherry-pick #25423 to 7.x: Do not use unversioned home with watcher binary path

[michalpristas]

Cherry-pick of PR #25423 to 7.x branch. Original message:

What does this PR do?

The problem is when agent tries to invoke watcher which checks upgraded agent for failures and at the same time unversioned home path is set.

Usually agent took its home path which is {top_path}/data/elastic-agent-{hash}/elastic-agent and invoked watch sub-command on binary at that path.

When unversioned path is set it tries to run watcher from {top_path}/data/elastic-agent and as so it results in binary not found.

Fixes: #25371

Why is it important?

Reduce noise created by failng attempt to run this. This is just a case when running container sub-command which tempers paths.

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.

cc @michel-laterman

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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• Build Cause: Pull request #25459 updated

• Start Time: 2021-05-04T09:15:55.443+0000

• Duration: 105 min 49 sec

• Commit: be15fde

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Failed	0
Passed	6733
Skipped	31
Total	6764

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Test	Results
Failed	0
Passed	6733
Skipped	31
Total	6764