github: Fix daily/release doc publish workflow

We now need pyelftools to build the docs.  See the following commit:

commit 83b346edefc37615d241de7f5b807b9bf96201d1
Author: Alexey Brodkin <abrodkin@synopsys.com>
Date:   Fri Feb 7 11:37:25 2020 +0300

    runners: opeocd: Allow loading Elf-files

Match the update to doc-build.yml to `pip3 install pyelftools`.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>