CI: add github workflow to run Coverity scans

CI: add github workflow to run Coverity scans

Add a workflow that performs a build like it's done by osstest
Coverity flight and uploads the result to Coverity for analysis. The
build process is exactly the same as the one currently used in
osstest, and it's also run at the same time (bi-weekly).

This has one big benefit over using osstest: we no longer have to care
about keeping the Coverity tools up to date in osstest.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>