fix(s001): the serve step prints the body and serve.log when GET / is not 200

Run 35518054546 (the package workflow dispatched for slice 013) printed
"HTTP 500 on port 36821" on linux and nothing else; the exception was in
serve.log, which the step only printed when there was no response at all.
The locally built binary answers 200 from its first request, so the log
is the only way to see what differs on the runner.

Signed-off-by: Ayla Croft <aylacroft@proton.me>

Author: HackTuah

.github/workflows/package.yml

@@ -252,6 +252,13 @@ jobs:
           echo "HTTP $code on port $PORT"
           echo "COLD_START_MS=$elapsed" | tee -a "$GITHUB_ENV"
           echo "### ${{ matrix.name }}: cold start to first HTTP 200: **${elapsed} ms**" >> "$GITHUB_STEP_SUMMARY"
+          # A non-200 says nothing on its own: run 35518054546 (slice 013) printed "HTTP 500"
+          # and nothing else, and the log that would have named the exception was never
+          # read. The body and the tail of serve.log go to stderr before the step fails.
+          if [ "$code" != "200" ]; then
+            echo "--- body of GET / ---" >&2; curl -sS "http://127.0.0.1:$PORT/" | head -c 2000 >&2; echo >&2
+            echo "--- serve.log ---" >&2; tail -60 serve.log >&2
+          fi
           test "$code" = "200"
           # Stopping it, and Windows needs its own verb. `kill` from Git-bash does not stop a
           # native Windows process, so the backgrounded .exe outlived the step and the job hung