ci: add Coverity to GHA

We need to run Coverity on a clang build, because Coverity's gcc
misreports its version causing ASS_DEPRECATED_ENUM version checks to
falsely suceed, which then causes the build to fail.
Coverity's clang likely also misreports its version, but whatever clang
build Coverity is using happens to work with our current code.
Coverity's gcc failing:
  https://travis-ci.org/github/libass/libass/builds/743655400
Coverity's clang succeeding:
  https://travis-ci.org/github/libass/libass/builds/740511824

Also Coverity's travis script is borked and always returns an error even
on success, because it expects Coverity's server to respond with code
201, but they actually return 200 instead.
To get meaningful job status we are using a custom script based on
Coverity's upstream script and fontforge's Coverity script.

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