cups: use BUILDROOT instead of DESTDIR

cups: use BUILDROOT instead of DESTDIR

The cups documentation is clear that the correct way to install into an
alternate root directory is to use the BUILDROOT variable. From INSTALL.md:

	Use the `BUILDROOT` variable to install to an alternate root directory:

	    make BUILDROOT=/some/other/root/directory install

DESTDIR works, but we should use the mechanism the project specifically
created for this purpose.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>