ci(e2e-ddil): install gotestsum from prebuilt tarball

`go install gotest.tools/gotestsum@v1.12.0` failed on the self-hosted
runner because v1.12.0 pulls golang.org/x/tools v0.19.0, whose
tokeninternal static-assert pegs a narrow Go window (~1.22). Both older
and newer runner Go versions trip `invalid array length -delta * delta`.
A prebuilt linux_amd64 binary sidesteps toolchain coupling entirely
without changing the gotestsum version pin.

.github/workflows/e2e-ddil.yml

@@ -54,11 +54,24 @@ jobs:
           path: mulga
 
       - name: Install gotestsum
+        env:
+          GOTESTSUM_VERSION: "1.12.0"
         run: |
-          # Idempotent: re-running `go install` on a cached binary is a no-op.
-          # Pinned tag keeps Actions output format stable across runs.
-          go install gotest.tools/gotestsum@v1.12.0
-          echo "${HOME}/go/bin" >> "${GITHUB_PATH}"
+          # Prebuilt tarball, not `go install`: gotestsum v1.12.0 pulls
+          # x/tools v0.19.0 whose tokeninternal static-assert pegs a
+          # narrow Go window (~1.22). The self-hosted runner's Go falls
+          # outside that window, so source-install fails with
+          # `invalid array length -delta * delta`. A prebuilt binary
+          # sidesteps toolchain coupling entirely.
+          BIN_DIR="${RUNNER_TEMP}/gotestsum-bin"
+          mkdir -p "${BIN_DIR}"
+          if [ ! -x "${BIN_DIR}/gotestsum" ]; then
+            curl -fsSL \
+              "https://github.com/gotestyourself/gotestsum/releases/download/v${GOTESTSUM_VERSION}/gotestsum_${GOTESTSUM_VERSION}_linux_amd64.tar.gz" \
+              | tar -xz -C "${BIN_DIR}" gotestsum
+          fi
+          echo "${BIN_DIR}" >> "${GITHUB_PATH}"
+          "${BIN_DIR}/gotestsum" --version
 
       - name: Build Install Artifacts
         working-directory: mulga/scripts/tofu-cluster