Ensure all installers have a shebang line to specify bash

Closes #134

install/asdcontrol.sh

@@ -1,3 +1,5 @@
+#!/bin/bash
+
 # Install asdcontrol for controlling brightness on Apple Displays
 if [ -z "$OMARCHY_BARE" ] && ! command -v asdcontrol &>/dev/null; then
   git clone https://github.com/nikosdion/asdcontrol.git /tmp/asdcontrol