ci(e2e-go): unbuffer sed in test pipeline for live log streaming

GNU sed defaults to block-buffered stdout when stdout is a pipe
(GitHub Actions runner). Accumulates 4-64KB before flushing, so
test output appeared in sporadic batches unaligned to test
boundaries. -u flag forces line-buffered output so each go test -v
line surfaces in the Actions UI as it is produced.

Side effect: none. No process count change, no SSH session change,
no test execution change. Pure output flushing on the runner host.

.github/workflows/e2e-go.yml

@@ -189,7 +189,7 @@ jobs:
                  ARTIFACT_DIR='${ARTIFACT_DIR}' \
                  ./\$SUITE.test -test.v -test.timeout=30m \
                  2>&1 | tee '${ARTIFACT_DIR}/test-'\$SUITE'.log' \
-                      | sed -E 's/^[[:space:]]+([a-zA-Z0-9_]+_test\.go:[0-9]+: )?//'; \
+                      | sed -uE 's/^[[:space:]]+([a-zA-Z0-9_]+_test\.go:[0-9]+: )?//'; \
                rc=\${PIPESTATUS[0]}; \
                echo \"::endgroup::\"; \
                if [ \$rc -ne 0 ]; then \