[28.x backport] Add escape hatch for GODEBUG=x509negativeserial

[vvoland]

• backport: Add escape hatch for GODEBUG=x509negativeserial #6371

- What I did
Negative serial number certificates are deprecated and Go has also since deprecated them. To use them with Go you need to pass in the GODEBUG=x509negativeserial=1 environment variable.

This PR allows setting the value inside docker context instead. This should then propagate environment variables to underlying plugins (such as buildx).

{
  "Name": "my-context",
  "Metadata": { "GODEBUG": "x509negativeserial=1" },
  "Endpoints": {
    "docker": { "Host": "unix:///var/run/docker.sock", "SkipTLSVerify": false }
  }
}

- How I did it

- How to verify it

- Human readable description for the release notes

Allow Docker CLI to set the `GODEBUG` environment variable when the key-value pair (`"GODEBUG":"..."`) exists inside the docker context metadata.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 55.26316% with 17 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/cli.go	55.26%	15 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[thaJeztah]

LGTM

sorry, looks like I missed that it could've used some squashing