fix: use proper shebang line

This is the most portable shebang line and should always be used.

etc/justfile

@@ -24,7 +24,7 @@ distrobox-ubuntu:
   distrobox create --image quay.io/toolbx-images/ubuntu-toolbox:22.04 -n ubuntu -Y
 
 setup-flatpaks:
-  #!/bin/bash
+  #!/usr/bin/env bash
   echo 'Installing flatpaks from the ublue recipe ...'
   flatpaks=$(yq '.flatpaks[]' < /usr/etc/ublue-recipe.yml)
   for pkg in $flatpaks; do \
@@ -62,7 +62,7 @@ nix-me-up:
   /usr/bin/ublue-nix-install
 
 update:
-  #!/bin/bash
+  #!/usr/bin/env bash
   echo "Staging system update..."
   rpm-ostree update