Fix download verification in snapshot builds

[andrewkroh]

What does this PR do?

Modify verifier to only delete the binary and .sha512 files when there is a
checksum mismatch. And only delete the .asc file when the signature does not
validate.

Fixes #252

Why is it important?

Downloaded artifacts fail signature validation in SNAPSHOT builds. You cannot test Packetbeat or Auditbeat with 8.2.0-SNAPSHOT builds.

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.

Related issues

[mergify]

This pull request does not have a backport label. Could you fix it @andrewkroh? 🙏
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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• Start Time: 2022-03-28T13:34:36.479+0000

• Duration: 19 min 2 sec

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[andrewkroh]

There is something weird with the linter failure on darwin CI.

Error: directive //nolint:staticcheck // crypto/openpgp is only receiving security updates. is unused for linter "staticcheck" (nolintlint)

If I remove the //nolint then the linter fails (on Linux) due to the same issue that nolint comment addresses. 🤷

[ph]

@andrewkroh look good to me, I will merge it.. Not sure about why the linter is so weird.

I'll make a note we need an integration test for this scenario. To be honest I hope with the changes of shippers and the beats we can remove the download on demande when features are need.

cc @nimarezainia, we will also need to add this feature to the artifact api so we can control the bandwith/speed over a network, if not we will have the same issues as an upgrade.