Use /usr/bin/env to find bash

This enables `npm test` to successfully call scripts in `bin/` with
the `bash` binary specified by `PROFILE`.

---

Additional information:

In addition to potentially preferring an alternative version of bash
than the one in `/bin/bash`, some distributions forego storing bash in
`bin/` at all, such as `NixOS`:

```bash
$ cat /etc/os-release | rg '^NAME=' -r ''
NixOS
$ which bash
/run/current-system/sw/bin/bash
```

bin/addons-linter.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/env bash
 
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this

bin/build-addon.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/env bash
 
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this

bin/commons.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/env bash
 
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this