ci: Rework CI for Darwin

This branch does a few things:

• Makes GHC on platforms which ship runtime-retargetable toolchains (e.g. Darwin) more robust by passing the --target=<triple> flag where possible (fixing #20162 (closed))
• Refactors the CI-phase timing logic into ci.sh
• Isolates the build from $HOME, using the CABAL_DIR environment variable to ensure that cabal-install doesn't look in $HOME
• Eliminates the usage of nix-shell from the CI infrastructure, rather generating a shell fragment to provide the environment (fixing #20131 (closed))
• Performs a few sundry cleanups