[ca] fix SC2015 for CI shellcheck 0.9

Unambiguous if-form for the two-condition guard. CI (node:20-bookworm,
Debian shellcheck 0.9) flags the A && B || C pattern where the koalaman
stable docker path passed. No behavior change.

CI evidence: https://git.knownelement.com/KNEL/ca/actions/runs/117821
Ticket: https://projects.knownelement.com/issues/784

docker/stepca-prod/init-stepca.sh

@@ -22,7 +22,9 @@ ROOT_KEY="/root/ca-root/root.key"
 IMAGE="smallstep/step-ca@sha256:e9e8fa3262bf37b130962ffddbf6a64ac188f0bbb80959cf3ddc04c6bf294c3d"
 TAILNET_IP="$(tailscale ip -4)"
 
-[ -f "$ROOT_CRT" ] && [ -f "$ROOT_KEY" ] || { echo "FAIL: fleet root not found at $ROOT_CRT/$ROOT_KEY" >&2; exit 1; }
+if [ ! -f "$ROOT_CRT" ] || [ ! -f "$ROOT_KEY" ]; then
+  echo "FAIL: fleet root not found at $ROOT_CRT/$ROOT_KEY" >&2; exit 1
+fi
 [ -n "$TAILNET_IP" ] || { echo "FAIL: no tailscale IP" >&2; exit 1; }
 if [ -d "$DATA" ] && [ "${1:-}" != "--reinit" ]; then
   echo "FAIL: $DATA exists (pass --reinit to wipe and redo)" >&2; exit 1