We depend on GNU-specific du switch `--apparent-size`. Busybox has this
implemented, but as `-b` instead.
Another part of the build system uses `cp --preserve=timestamps`. This
can be replaced by `rsync -t`, and rsync is already used through the
file.
Thanks to those changes, Serenity can be built on a Busybox system,
without GNU coreutils.
* `chmod -x` as it's for sourcing, not for executing
* Remove run line, for the same reason
* Rename it from .shell_include.sh to shell_include.sh, since e.g.
`rg` doesn't search in hidden files by default
No behavior change.
