Fall back to os.Get[ug]id if user.Current() fails

[david-kow]

What does this PR do?

In the case of user.Current() call failing with user.UnknownUserIdError we fallback to os.Getuid() and os.Getgid() to populate uid and gid values respectively.

Why is it important?

#28265 added populating username, uid and gid for diagnostic purposes. This information is taken from password database (eg. /etc/passwd file) based on uid. Unfortunately, in some cases an entry for current uid is not present there. This is the case in Kubernetes when user sets uid of their choice. This is currently breaking ECK E2E tests for 7.16.0-SNAPSHOT.

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.

How to test this PR locally

Run the agent normally then use: elastic-agent diagnostics

Related issues

• Relates Add start of elastic-agent diagnostics command #28265.

All credit goes to @barkbay 🙇

[mergify]

This pull request does not have a backport label. Could you fix it @david-kow? 🙏
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 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[mergify]

This pull request does not have a backport label. Could you fix it @david-kow? 🙏
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 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2021-10-28T11:25:29.008+0000

• Duration: 125 min 8 sec

• Commit: 33b8da5

Test stats 🧪

Test	Results
Failed	0
Passed	54138
Skipped	5359
Total	59497

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[michel-laterman]

lgtm