Update Actions runner image

[wnienhaus]

Github has deprecated the ubuntu-18.04 image for running Github Actions with (see https://github.com/actions/runner-images/issues/6002).

We will need to ensure our build works with newer Ubuntu versions.

(Last time I tried, the binutils-esp32 build process did not work on ubuntu versions later than 18.04, so this might be more effort than just changing the image name 😞).

[wnienhaus]

Found this approach (i.e. using gcc 7) to building on ubuntu-20.04 and it works.

It does not work on ubuntu-22.04 because it doesn't have gcc-7 anymore, but building on 20.04 should hopefully last for quite some time before Github will deprecate that one.