ci: bump Go to 1.26.4 in docker test image workflow

Align the docker-test-image build test with go.mod (1.26.4) and the
other workflows / Dockerfile, which were already on 1.26.4.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lin-snow

.github/workflows/docker-test-image.yml

@@ -62,7 +62,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v6
         with:
-          go-version: "1.26.2"
+          go-version: "1.26.4"
           cache-dependency-path: "**/go.sum"
 
       - name: Set Go proxy