base-files: sysupgrade: avoid cat missing conffiles_static

base-files: sysupgrade: avoid cat missing conffiles_static

If the user removes all /lib/apk/packages/*.conffiles* files to prevent
sysupgrade from preserving configuration, the glob no longer matches and
sysupgrade ends up calling cat on a non-existent path:

  cat: can't open '/lib/apk/packages/*.conffiles_static': No such file or directory

Fix this by using find cmd.

Signed-off-by: Nick Hainke <vincent@systemli.org>
Link: https://github.com/openwrt/openwrt/pull/22071
Signed-off-by: Robert Marko <robimarko@gmail.com>