merge env vars during pod update

[vavo]

E-268: merge env vars during pod update

• resolves pod update --env replaces all env vars instead of merging #268 by merging runpodctl pod update --env values into the pod's existing env map instead of replacing the whole set.
• maintains full legacy command compatibility (stderr warnings only, stdout unchanged).
• keeps runpodctl as the primary binary; drop-in upgrade for existing users.
• preserves existing env keys such as PUBLIC_KEY while allowing provided keys to be updated in place.
• adds unit coverage for env parsing and merge behavior.

Test plan:

• go test ./...
• go test -tags e2e ./e2e
• live pod create/update/get/delete verification

[vavo]

live verification completed with a real Runpod API key.

verified flow:

• build runpodctl
• create gpu pod with:
  • --gpu-id 'NVIDIA GeForce RTX 4090'
  • --image ubuntu:22.04
  • --env '{"MY_VAR":"hello","OTHER_VAR":"world"}'
• update the pod with --env '{"MY_VAR":"updated"}'
• fetch the pod and verify:
  • MY_VAR=updated
  • OTHER_VAR=world is preserved
  • PUBLIC_KEY is preserved
• delete pod immediately after verification

result: pass

pod id used for verification: ajwu26z75znda9
cleanup completed successfully.

[promptless]

Promptless prepared a documentation update related to this change.

Triggered by runpod/runpodctl#269

Updated the --env flag documentation for runpodctl pod update to clarify that environment variables are now merged instead of replaced, preserving existing keys while updating specified values.

Review at https://app.gopromptless.ai/suggestions/f56234fa-1561-4c43-9451-cd7e5a4b78c9