CI: Test on MacOS and Linux ARM64 / Aarch64#524

Fix #372 #523, supercedes #374 This PR completely revamps the CI: - It provides an install scripts that is able to fetch releases from Nim website, nightly builds from Github or build from source, with versioning support for all 3 cases: - Previous code used https://github.com/alaviss/setup-nim which only support nightlies, but MacOS nightlies have been pending forever https://github.com/nim-lang/nightlies/pull/82 and blocking #374. - https://github.com/jiro4989/setup-nim-action does not support versioning for nightlies - It updates actions/cache to v4 as v2 is decommissioned on Feb 1, 2025, see #523 - It deletes the TEST_LANG env variable that was used to test c and cpp backend. We only test c, which is more widely used. As Constantine is pure Nim code and does not import C++ or use C unions or flexible array members we don't expect incompatibility with C++ backend (unless Nim upstream has a showstopper bug in atomics).