ci: harden Remove Snapshots step against silent failure

From a Gemini CI review: `find ... | xargs rm -rf` masks a find failure
(xargs exits 0 on empty stdin), and GNU xargs also runs `rm` once with no
operands on empty input. Add `set -o pipefail` so a find failure fails the
step, and `xargs -r` so rm is skipped when there is nothing to remove.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>