buildroot

samba4: ensure that copied cache.txt is writable

If the Buildroot tree is read-only, then cache.txt is copied read-only into
the build directory, and the configuration step fails.  Fix this in the
same way we do in other places, by opening permissions as we copy the file
using $(INSTALL).

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>