benchmark: fix shellcheck issues

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

benchmark

@@ -1,12 +1,14 @@
 #!/bin/bash
 
-tmpfile=$(mktemp /tmp/random-file.XXXXXX)
-dd if=/dev/urandom of=$tmpfile bs=$1 count=1
+BLOCK_SIZE=${1:-1G} # gigabyte by default
+OUT="${OUT:-$(mktemp /tmp/random-file.XXXXXX)}"
+
+dd if=/dev/urandom of="$OUT" bs="$BLOCK_SIZE" count=1
 
 go build ./cmd/tzsum || exit 1
 
 for impl in avx avx2 generic; do
 	echo $impl implementation:
-	time ./tzsum -name $tmpfile -impl $impl
+	time ./tzsum -name "$OUT" -impl $impl
 	echo
 done