Mock external services in libcdb CI tests

[peace-maker]

The libcdb module relies on multiple external services in the tests.

• https://gitlab.com/libcdb/libcdb
• https://libc.rip/
• https://debuginfod.elfutils.org/

They can each individually fail and take the CI with them. Maybe mock their API locally so the tests won't fail if one of them goes down.

The debug symbols can be cached using @actions/cache and a local simple proxy serving the cached file if it exists or passes the request on to the real service instead.

I'm not sure about the other ones.