tools/build: fix bug for documentation build script

This patch fixes a bug where the host machine python dependencies were
used instead of the ones from the virtual environment created by the
script.

Issue-Id: SCM-3032
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Change-Id: I411eb67039364f6224107cada3870f89c6f2ce40