Use #!/usr/bin/env bash in test scripts

This uses the path supplied `bash`, and will also allow the scripts to
run on NixOS where `/usr/bin/env` is shimmed for compatibility but
`/bin/bash` can't be relied on.

contrib/lint.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 cargo clippy --all-targets --keep-going --features=send,receive -- -D warnings