fix: disable the Green Tea GC experiment (GOEXPERIMENT=greenteagc)

[Isteb4k]

Description

Disable the Green Tea GC experiment (GOEXPERIMENT=greenteagc) that is baked
into the container-factory ALT golang toolchain when building KubeVirt Go
binaries. A single export GOEXPERIMENT=nogreenteagc is added to the build
setup step in images/virt-artifact/werf.inc.yaml, so it applies to all
KubeVirt binaries (notably virt-handler and virt-launcher, which host the
live-migration TLS proxy).

This drops only the GC experiment; the toolchain version and the ALT base image
(and therefore the CVE mitigation from cc5aff898) are left untouched.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: vm
type: fix
summary: "Restore live-migration throughput by disabling the Green Tea GC experiment in the KubeVirt build toolchain."