GH Workflows: Create CI job for Coverity scan

[moshekaplan]

I've noticed that https://scan.coverity.com/projects/net-snmp already exists, but appears to be only updated on an ad-hoc basis.

This PR adds a GitHub Action-based CI job to create a new Coverity build daily, so that there is always a recent scan available.

Note: Even a daily scan is within the limit, as Net-SNMP is ~750K LOC and per https://scan.coverity.com/faq :

Up to 14 builds per week, with a maximum of 2 build per day, for projects with 500K to 1 million lines of code

Before this is merged in, two new secrets will need to be created:

• COVERITY_SCAN_EMAIL with the email address used for accessing Coverity
• COVERITY_SCAN_TOKEN with the Project token from https://scan.coverity.com/projects/net-snmp?tab=project_settings

[bvanassche]

I couldn't get this working so I implemented an alternative myself. Feel free to take a look at commit d28d6a2.

[moshekaplan]

If you have something that works, then there's no need for this PR anymore. Thank you!