fix: finalize weekly postgres backup retention

infra/install-postgres-backup-cron.sh

@@ -2,8 +2,9 @@
 set -eu
 
 temporary=$(mktemp)
+app_dir=$(CDPATH= cd -- "$(dirname "$0")/.." && pwd)
 trap 'rm -f -- "$temporary"' EXIT HUP INT TERM
 
 crontab -l 2>/dev/null | grep -v 'postgres-backup.sh' > "$temporary" || true
-printf '%s\n' '30 3 * * 0 flock -n /tmp/miem-postgres-backup.lock /srv/miem_workers/infra/postgres-backup.sh >/dev/null 2>&1' >> "$temporary"
+printf '%s\n' "30 3 * * 0 flock -n /tmp/miem-postgres-backup.lock $app_dir/infra/postgres-backup.sh >/dev/null 2>&1" >> "$temporary"
 crontab "$temporary"