ci: dump regression.diffs on installcheck failure in the build image

[dimitri]

Problem

When RUN pg_virtualenv ... make -C src/monitor/ installcheck fails during the Docker build stage, the container running it is discarded on failure — so src/monitor/regression.diffs, the one file that actually shows what differed between expected and actual output, never makes it into the CI log. All CI (and anyone reading it later) gets is the postmaster log tail, which is often noise from unrelated concurrently-registered test fixtures and rarely points at the real cause.

This came up directly while diagnosing recent regress failures on #1149/#1151: pinning down the actual cause required reproducing the build locally and temporarily hacking the Dockerfile to cat the diffs file. That workaround is worth keeping permanently instead of redoing it by hand every time.

Fix

RUN pg_virtualenv -v ${PGVERSION} \
      -o "shared_preload_libraries=pgautofailover" \
      make -C src/monitor/ installcheck || \
      (find src/monitor -name '*.diffs' -exec cat {} +; exit 1)

On failure, find and cat every *.diffs file under src/monitor/ (covers both the REGRESS and ISOLATION suites, wherever pg_regress/pg_isolation_regress happen to write them), then re-raise the original failure so the build still fails exactly as before.

Verified

make build-pg18:

• Success path (current main) is unaffected — clean build, no output changes.
• Failure path: deliberately corrupted expected/create_extension.out's output locally, rebuilt, confirmed the diff appears directly in the build log (both in the final failure dump and in buildx's error-context excerpt), then reverted the deliberate corruption.