make shebangs posix compliant (#2974)

Previously, these scripts wouldn't work on exotic systems such as NixOS.

```
fd '\.sh$' -t f --exec sed -i 's@#!/bin/bash@#!/usr/bin/env bash@'
```

scripts/query_testing/api_benchmark.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 # By default, this script runs against `http://127.0.0.1:8536`, but you can pass a different Lemmy instance,