Fix vanilla-first-setup-prepare-files permission

[D3vil0p3r]

Providing vanilla-first-setup-prepare-files exec grant to allow

first-setup/vanilla_first_setup/utils/processor.py

Line 65 in d055de2

subprocess.run([pkexec_bin, "/usr/bin/vanilla-first-setup-prepare-files", getpass.getuser()]) # type: ignore

to be run correctly when first-setup is run as non-root.

If it is not done, it will produce a permission denied error when opening in write mode the commands_script_path = "/etc/org.vanillaos.FirstSetup.commands" file.

[mirkobrombin]

LGTM but I wait for a 2nd reviewer