CI: Test on MacOS and Linux ARM64 / Aarch64

[mratsim]

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 add m1 matrix nim-lang/nightlies#82 and blocking CI: Github MacOS now uses ARM64.  #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 Upstream CI updates: Linux ARM, cache versions deprecation #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).