file: return wrapper to fix builds when file is in buildtools-tarball

Wrapper that adds proper path to magic file was removed in
47db876d in favor of
environment-setup.d/file.sh which now contains `MAGIC` environment variable.
This breaks builds that rely on external buildtools with `file` included in
buildtools-tarball (e.g. via TOOLCHAIN_HOST_TASK) because do_populate_sysroot
task invokes `file` via `subprocess.check_output` (in is_elf function) and
doesn't use variables from files in environment-setup.d. To address this use
case, we need to preserve the wrapper, but also have
environment-setup.d/file.sh for the purposes described in
47db876d.

Signed-off-by: Oleksandr Hnatiuk <ohnatiuk@cisco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>