Fix linux builds

[acunniffe]

x86 Linux builds have been broken (really sorry all!), mainly due to some stupid cache logic and a lib we included libc to help us do exit signals properly and pass more of the git compatibility tests.

• Cache fixed and each binary tested in containers
• Moved the build container back to Ubuntu 20 (5 years old) to maximize compatibility for libc. Later versions will work fine (and I've tested them). Earlier versions may not.
• Used a GitHub Public Runner with arm support to build ARM. Thanks GH for paying for our CI!

Still to do #142

[git-ai-cloud-dev]

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