scripts: fix the unbound local that killed the sweep at its first cell

`local b=$1 ub=$2 dir="$OUT/b${b}_ub${ub}"` expands every argument of the builtin
before the builtin assigns any of them, so $b was unbound and set -u aborted the run
four seconds in - after the service was already stopped. Two statements instead.

Found by running it under systemd-run: it exited non-zero with no cells run, which is
exactly what the per-cell TSV is for.

scripts/bench-batch-ubatch.sh

@@ -74,7 +74,10 @@ service_start() {
 trap service_start EXIT
 
 run_cell() {
-    local b=$1 ub=$2 dir="$OUT/b${b}_ub${ub}" sampler start rc wall
+    # two statements: `local a=1 b=$a` expands b before the builtin assigns a, and
+    # set -u turns that into "b: unbound variable"
+    local b=$1 ub=$2
+    local dir="$OUT/b${b}_ub${ub}" sampler start rc wall
     mkdir -p "$dir"
     if [ -s "$dir/bench.json" ]; then
         echo "b=$b ub=$ub: bench.json present, skipping" >&2