feat(ci): read Go version from go-versions.yml for system-tests weblog build

Read the stable Go version from the local go-versions.yml (checked out at
binaries/dd-trace-go) and pass it as GO_VERSION to build.sh, which forwards
it as --build-arg GO_VERSION to docker buildx build.

This wires dd-trace-go's single source of truth into the system-tests
weblog Docker build, so Go version bumps in go-versions.yml automatically
propagate to the container image without a manual PR.

Companion PR: DataDog/system-tests#6350

Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>

Author: kakkoyun

.github/workflows/system-tests.yml

@@ -167,8 +167,23 @@ jobs:
           ref: ${{ inputs.branch_ref || github.ref }}
           path: 'binaries/dd-trace-go'
 
+      - name: Read Go versions
+        id: go-versions
+        shell: bash
+        run: |
+          file="binaries/dd-trace-go/go-versions.yml"
+          stable=$(grep '^stable:' "$file" | sed 's/.*"\(.*\)".*/\1/')
+          if [[ -z "$stable" ]]; then
+            echo "ERROR: Failed to parse stable version from ${file}" >&2
+            exit 1
+          fi
+          echo "stable=${stable}" >> "$GITHUB_OUTPUT"
+          echo "Go stable version: ${stable}"
+
       - name: Build weblog
         run: ./build.sh -i weblog
+        env:
+          GO_VERSION: ${{ steps.go-versions.outputs.stable }}
 
       - name: Build runner
         uses: ./.github/actions/install_runner