build: disable the jsonv2 GOEXPERIMENT

Go 1.27 enables the jsonv2 experiment by default, reimplementing
encoding/json and making json.RawMessage an alias for jsontext.Value. The
repo inherited this from #122, which bumped the go directive from 1.26.6 to
1.27.0 -- that directive is a minimum toolchain requirement, so GOTOOLCHAIN
=auto then builds everything with go1.27 and its default experiments.

This emulator serializes customer Lambda payloads, so swapping JSON
implementation should be a deliberate, tested decision rather than a side
effect of a toolchain bump. Pin it off for now.

There is no GODEBUG opt-out -- jsonv2 is selected at compile time via build
tags -- so it must be an environment variable. Exporting it from the
Makefile covers local builds, the docker targets (which re-enter this
Makefile in the container) and CI, which invokes only make targets.

Fixes the localstack branch, red since Aug 22, without touching the
upstream test that the experiment broke.

Author: carole-lavillonniere

Makefile

@@ -1,10 +1,31 @@
 # LOCALSTACK CHANGES 2022-03-10: remove linker flags and add gc flags for delve debugger
 # LOCALSTACK CHANGES 2022-03-28: change compile src folder
 # LOCALSTACK CHANGES 2022-11-14: add --rm flag to compile-with-docker
+# LOCALSTACK CHANGES 2026-08-31: disable the jsonv2 GOEXPERIMENT (see GOEXPERIMENT below)
 
 # RELEASE_BUILD_LINKER_FLAGS disables DWARF and symbol table generation to reduce binary size
 #RELEASE_BUILD_LINKER_FLAGS=-s -w
 
+# Pin encoding/json to the v1 implementation.
+#
+# Go 1.27 enables the jsonv2 GOEXPERIMENT by default. It reimplements
+# encoding/json and, among other things, makes json.RawMessage an alias for
+# jsontext.Value. This emulator serializes customer Lambda payloads, so
+# changing JSON implementation is a behavioural change that should be adopted
+# deliberately and tested -- not inherited from a toolchain bump. It arrived
+# via #122, which moved the go directive from 1.26.6 to 1.27.0; that directive
+# is a minimum toolchain requirement, so GOTOOLCHAIN=auto then builds
+# everything with go1.27 and its default experiments.
+#
+# There is no GODEBUG opt-out -- jsonv2 is selected at compile time via build
+# tags -- so this has to be an environment variable on every go invocation.
+# Exporting it here covers local builds, the docker targets (which re-enter
+# this Makefile inside the container) and CI, which runs only make targets.
+# Accepted by go1.26 as well, so it is safe on older toolchains.
+#
+# Remove when adopting jsonv2 is a deliberate, tested decision.
+export GOEXPERIMENT := nojsonv2
+
 BINARY_NAME=aws-lambda-rie
 ARCH=x86_64
 GO_ARCH_x86_64 := amd64