ci(pool-test): fix scheduled runs and read env from secrets or vars
workflow_dispatch inputs are empty for schedule events, so the scheduled
pool test built a broken package URL (--version "") and failed preflight.
Fall back to the latest nightly deb (R2) when no version/package_url input
is given, default the thresholds/duration/pools, and default cleanup to
enabled. Also read RUSTFS_API_ENDPOINT / RUSTFS_NODES / RUSTFS_SSH_USER
from secrets first (variables as fallback) so either configuration works.