fix(build): derive xgo image tag from GO_VERSION

[manusa]

Problem

The Linux job on #419 fails while cross-compiling the Windows DLL:

x86_64-w64-mingw32-ld: /tmp/go-link-*/export_file.def:1: syntax error
x86_64-w64-mingw32-ld: /tmp/go-link-*/export_file.def: file format not
  recognized; treating as linker script
collect2: error: ld returned 1 exit status

helm-windows-4.0-amd64.dll is therefore never produced, and the build later
stops on:

Rule 1: RequireFilesExist failed with message:
  .../native/out/helm-windows-4.0-amd64.dll

The enforcer error is the symptom that gets reported, not the cause.

Root cause

The Go version is pinned in three places, and only two were updated when
GO_VERSION moved to 1.26.7:

Pin	Value
native/go.mod	go 1.26.0 (raised by #419)
GO_VERSION in .github/workflows/*.yml	1.26.7
xgo image tag in Makefile	go-1.25.10

Once go.mod requires >= 1.26.0, the Go toolchain inside the go-1.25.10
container auto-downloads Go 1.26.0 and links with it — against the mingw-w64
binutils that image ships. That older GNU ld cannot parse the .def file Go
1.26 emits.

This is why main stayed green: its go.mod still says 1.25.10, so the
container never downloads a different toolchain and the mismatch never appears.

Fix

Derive the image tag from GO_VERSION. The workflows already export it, and
make's ?= prefers an environment value over the default, so CI and the
Makefile can no longer drift apart. Local builds default to 1.26.7.

Verification

Ran the exact failing scenario inside ghcr.io/techknowlogick/xgo:go-1.26.7:

• ships go1.26.7 and LLD 21.1.8 as x86_64-w64-mingw32-ld (not GNU ld)
• no toolchain download occurs, since 1.26.7 already satisfies the directive
• a -buildmode=c-shared windows/amd64 build links successfully

Confirmed with go.mod at both go 1.26.0 (as on #419) and go 1.25.10 (as on
main today), so this is safe to land ahead of #419.

Unblocks #419, the last remaining dependabot PR.

[Copilot]

🟢 Approval recommended

The change is small, self-contained in the Makefile, and correctly ties the xgo image tag to the already CI-exported GO_VERSION to eliminate a known drift failure mode.

Pull request overview

Aligns the xgo Docker image selection with the CI-pinned Go toolchain version to prevent Makefile/CI drift that can break cross-compilation (notably the Windows DLL build) when native/go.mod requires a newer Go than the container ships.

Changes:

• Introduces GO_VERSION ?= 1.26.7 as a Makefile default that is overridden by CI’s exported GO_VERSION.
• Derives XGO_IMAGE from GO_VERSION and switches the cross-platform build to use -image $(XGO_IMAGE).

File summaries

File	Description
Makefile	Adds GO_VERSION/XGO_IMAGE variables and uses them for xgo cross-platform builds to keep CI and Makefile in sync.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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