Nested exec expressions like ${execp echo '${execp echo hi}'} are not
evaluated correctly because parsing expressions like these generates and
registers new callbacks but we never wait for these callbacks to
complete before returning the result of the evaluation.
Fix this by re-evaluating expressions after running all pending
callbacks.
- use sccache for build caching
- add coverage reports as build artifacts
- add lcov-summary for coverage summary
- clean up/refactor CI yaml
This resolves the FreeBSD part of #754.
