doc/sphinx: Force sphinx to re-build everything as to avoid stale docs

With the introduction of the 'docs' target we make sure that we always
build the documentation, but currently breathe is unaware of the fact
that the doxygen XML database was changed in between runs. It is obvious
when changing only source code not rST bits.

This patch makes sphinx ignore the saved environment and always
rebuild the docs.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>