Modifying default variables and config

Renamed variables for defaults:
- flatpak_method -> default_flatpak_method: system
- defaults.paths.prefix -> default_install_prefix: /usr/local
New variables:
- default_install_method: package

tasks/helpers/cargo_build.yml

@@ -50,6 +50,7 @@
                 group: root
                 mode: "0755"
                 state: directory
+
             - name: Copy extra files
               loop: "{{ install_files | dict2items }}"
               loop_control: