Cherry-pick #22322 to 7.x: Fix incorrect hash when upgrading agent (#22322)

[michalpristas]

Cherry-pick of PR #22322 to 7.x branch. Original message:

What does this PR do?

For some weird special case when elastic-agent sha512 file contains a filename prefixed with ./
e.g:

748e927284f8eac2f5d8724f85a3be34d271b207147024906be4d254c5ff60affc51b191b7895e53bc3a1f4d1e76ef72e97c993d10b4be4804d9d6065ffefb9e  ./elastic-agent-7.11.0-SNAPSHOT-darwin-x86_64.tar.gz

elastic agent evaluates hashes incorrectly because it just trims the filename suffix. agent does not consider ./ to be part of the filename so it suspects that hash is
748e927284f8eac2f5d8724f85a3be34d271b207147024906be4d254c5ff60affc51b191b7895e53bc3a1f4d1e76ef72e97c993d10b4be4804d9d6065ffefb9e ./ instead of
748e927284f8eac2f5d8724f85a3be34d271b207147024906be4d254c5ff60affc51b191b7895e53bc3a1f4d1e76ef72e97c993d10b4be4804d9d6065ffefb9e

This PR just finds a correct line in a file and takes hash which is a fixed size string anyway without trimming suffixes.

Why is it important?

Fixes: #22306

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.

[elasticmachine]

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

[blakerouse]

Backport looks good.

[elasticmachine]

💚 Flaky test report

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Test	Results
Failed	0
Passed	1394
Skipped	4
Total	1398

[elasticmachine]

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• Build Cause: [Pull request #22323 opened]

• Start Time: 2020-10-30T18:04:25.548+0000

• Duration: 36 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1394
Skipped	4
Total	1398