Use #!/usr/bin/env bash instead of #!/bin/bash (#254)

Refer to https://github.com/dylanaraps/pure-bash-bible#shebang

polkadot/parachain/test-chains/basic_add/build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # Make LLD produce a binary that imports memory from the outside environment.

polkadot/parachain/test-chains/build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 rm -rf ./target