php: delete /usr/lib/php/build instead of /usr/lib/php

Fixes #8121

External PHP modules like the OPcache may live in
/usr/lib/php/extensions, so only delete /usr/lib/php/build

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>