ci: Coverity Scan

[kare]

This PR requires GitHub Actions project secrets to be set for:

• COVERITY_SCAN_EMAIL
• COVERITY_SCAN_TOKEN
  Find token in the project's "Project Settings" tab under "Project token" on the Coverity Scan website.
  https://scan.coverity.com/dashboard

[dancrossnyc]

This sounds great on the face of it (though for some reason I thought we had coverity scanning; maybe I imagined that, or perhaps it has atrophied to the point of being non-functional). However, the job itself seems to fail unpacking a tarball, and I'm not in a great position to debug it.

[kare]

Coverity Scan was enabled when Travis was used for CI.

I think it fails because coverity token is not set?
Coverity Scan GH Action tries to download a .tar.gz file but loads an empty file.

curl https://scan.coverity.com/download/cxx/linux64 \                                                                
    --no-progress-meter \
    --output cov-analysis.tar.gz \
    --data "token=${TOKEN}&project=9fans%2Fplan9port"

So Coverity Scan needs to be connected to plan9port from https://scan.coverity.com/dashboard_np by clicking add new project button or alternatively add Coverity to Github project via GitHub settings page as an application.
I'm unable to test it in my own repository as it seems that Coverity Scan only supports GitHub Organisations and not individual users.

[dancrossnyc]

Sigh; ok. I might need to drag Russ in for this; I'm pretty sure he's got all the creds for the org.

[kare]

Coverity seems to be in use by drawterm project also
https://github.com/9fans/drawterm/blob/main/.github/workflows/coverity.yml