Get CI working on Linux again

[BartmanAbyss]

Ubuntu 20 is apparently deprecated.
Also, do we know why there's a elf2hunk CI runner in this repo (that was disabled manually) as well as in the elf2hunk repo?

[grahambates]

Hopefully fixed in 1740d0e. Not sure why we have a duplicate elf2hunk runner. I see it was added by @petmac along with the binutils-gdb-gcc one.

[petmac]

Feel free to delete it if it’s not needed.

[BartmanAbyss]

I think it's fine to leave it in.

[BartmanAbyss]

@grahambates thanks. seems the CI runs completed successfully. Can you integrate the artifacts into the tree and check if they work?
Then I could release a new version.