Overwrite files with format: ignition

This aligns with the default cloud-init behavior and enables e.g
overwriting distro-default files via RKE2ConfigTemplate using the
spec.template.spec.files API.

bootstrap/internal/ignition/butane/butane.go

@@ -95,6 +95,7 @@ storage:
       {{ if ne .Permissions "" -}}
       mode: {{ .Permissions }}
       {{ end -}}
+      overwrite: true
       contents:
         {{ if eq .Encoding "base64" -}}
         inline: !!binary |