Add workflow for clang & clang++

[ribose-jeffreylau]

Currently, only gcc and g++ are tested.

Adding coverage for clang and clang++ would better align with tests in https://github.com/rnpgp/rnp/blob/d440935cd257457afc865413070f7b4c4373c535/ci/env-common.inc.sh#L14 .