snapshot/downloader: fix fetching snapshot URI

[belimawr]

What does this PR do?

When parsing the response from the artifacts-api to get the latest snapshot (https://artifacts-api.elastic.co/v1/search/%s-SNAPSHOT/elastic-agent) it could happen that an entry that is not from the Elastic-Agent (like the Elastic-Agent-Shipper) would be evaluated and it would cause the snapshotURI function return an error.

This PR fixes it by iterating the whole map before returning an error.

Why is it important?

It fixes a bug

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

Manual testing

1. Build the 8.6 branch (it will build the version 8.6.0)
2. Build the main branch with SNAPSHOT=true (it will build the version 8.7.0-SNAPSHOT)
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. Copy the built artefacts to the drop path
9. Restart the Elastic-Agent
10. 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-SNAPSHOT -v -e -d "*"
    

The caveat here is that because the bug depends on the order the API response is iterated on, sometimes it just works.

Automated version

1. Grab my unittest file: https://github.com/belimawr/elastic-agent/blob/4e7a3b7ee3a011e4f8ac43c66984bc789448c251/internal/pkg/agent/application/upgrade/upgrade_integration_test.go
2. Run it.

The same problem as testing manually, it depends on the order of iteration on the map, so it was an intermittent bug.

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

[elasticmachine]

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• Start Time: 2022-11-30T08:49:51.191+0000

• Duration: 16 min 7 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4617
Skipped	13
Total	4630

💚 Flaky test report

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.333% (59/60)	👍
Files	69.082% (143/207)	👍
Classes	69.133% (271/392)	👍
Methods	53.8% (814/1513)	👍
Lines	38.976% (8791/22555)	👎 -0.013
Conditionals	100.0% (0/0)	💚

[cmacknz]

@belimawr let's get this merged so it can be included in the next 8.6 build candidate