Switch format_json userscript to /bin/sh again

It was switched to bash in 22e4a800a1 for
"-o pipefail", but doesn't actually use pipes, so we might as well
switch it back to sh again.

(cherry picked from commit aa333512a1)

misc/userscripts/format_json

@@ -1,5 +1,5 @@
-#!/usr/bin/env bash
-set -euo pipefail
+#!/bin/sh
+set -eu
 #
 # Behavior:
 #   Userscript for qutebrowser which will take the raw JSON text of the current