php.buildEnv: Don't inherit dev from the original php

mkDerivation uses the dev output in buildInputs if it exits, hence the
php-with-extensions package was never built or put into the path of
packages dependent on it during build. With this fix, the php packages
built with buildEnv or withExtensions don't have any dev outputs;
packages which need the dev output can refer to the phpXXbase packages
instead.