fix: prevent /etc/ merge issues (#43)

* fix: prevent /etc/ merge issues
discussed at length in #28
also including tons of comments about the issue

* fix: change references of /etc/ to /usr/etc

etc/justfile

@@ -26,7 +26,7 @@ distrobox-ubuntu:
 setup-flatpaks:
   #!/bin/bash
   echo 'Installing flatpaks from the ublue recipe ...'
-  flatpaks=$(yq '.flatpaks[]' < /etc/ublue-recipe.yml)
+  flatpaks=$(yq '.flatpaks[]' < /usr/etc/ublue-recipe.yml)
   for pkg in $flatpaks; do \
       echo "Installing: ${pkg}" && \
       flatpak install --user --noninteractive flathub $pkg; \