Use #!/usr/bin/env everywhere for local testnets (#3606)

Full local testnet support for people that don't have `/bin/bash`

scripts/local_testnet/clean.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 #
 # Deletes all files associated with the local testnet.

scripts/local_testnet/dump_logs.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Print all the logs output from local testnet