Use a "bash" shell from PATH

In case of nix environment we may be using bash from non-standard path.

bench.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 #------------------------------------------------------------------------------
 # Benchmark groups

bin/bench-exec-one.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Environment passed:
 # BENCH_EXEC_PATH: the benchmark executable