github actions

[r00t-]

fix #457

[mbehr1]

previously we did run build checks towards older gcc versions (as some e.g. raspberry distros have quite old gcc.) Did you remove it on purpose?

[mbehr1]

regarding leptonic: we could use e.g. https://github.com/actions/cache to speed up long build times.

[r00t-]

@mbehr1:
i did not remove the gcc versions on purpose, at least not as much as leptonic,
i only did not re-add them to the from-scratch implementation.
i'd say this is a basic implementation, feel free to make MRs to add stuff back in.

i've also been wondering if gitlab actions can store artifacts, so we could provide binaries for
the part of our users that are not capable of compiling (branches) themselves.