Use only fs.NewDownloader for file://.

[blakerouse]

What does this PR do?

Changes the upgrade artifacts fetcher to only use the fs.NewDownloader when a --source-uri is passed with file:// prefix.

Why is it important?

Ensures that when a file:// source URI is provided that a file will never be downloaded from the artifacts API. Upgrade requested to use a local file it should only look there.

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/fragments using the changelog tool
• I have added an integration test or an E2E test (already used in tests)

How to test this PR locally

$ elastic-agent upgrade --skip-verify --source-uri file:///home/doesnt-exist 8.12.0-SNAPSHOT
Error: Failed trigger upgrade of daemon: failed download of agent binary: unable to download package: package '/home/ubuntu/elastic-agent-8.12.0-SNAPSHOT-linux-arm64.tar.gz' not found: open /home/ubuntu/elastic-agent-8.12.0-SNAPSHOT-linux-arm64.tar.gz: permission denied
For help, please see our troubleshooting guide at https://www.elastic.co/guide/en/fleet/8.12/fleet-troubleshooting.html

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

[elasticmachine]

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• Start Time: 2023-11-06T14:40:23.037+0000

• Duration: 50 min 23 sec

❕ Flaky test report

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[michalpristas]

looks good

[michalpristas]

you can even move this to fs and have factory = fs.NewDownloaderFactory to make it a bit more readable

[blakerouse]

i didn't do that as the factory interface is private, takes arguments that the download module doesn't really need to import as its not used

[blakerouse]

buildkite test this

[cmacknz]

Buildkite failure is only in the merge coverage reports step, force merging.