[gardening] Use '#!/usr/bin/env bash' instead of '#!/bin/bash'

Before this commit:

```
$ git grep '#!/usr/bin/env bash' | wc -l
10
$ git grep '#!/bin/bash' | wc -l
1
```

After this commit:

```
$ git grep '#!/usr/bin/env bash' | wc -l
11
$ git grep '#!/bin/bash' | wc -l
0
```

Author: practicalswift

utils/build-toolchain

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # utils/build-toolchain - documents process for building a toolchain
 #