CI: Use a script to generate the build matrix

[RyanGlScott]

Currently, we copy-paste a nearly-identical build matrix across five different CI workflows, which is tedious to maintain. It would likely save us time in the long run to instead generate build matrices using JSON, as described in this GitHub Actions documentation. We've already done something similar for what4 here.

[langston-barrett]

There are also repeated bits of setup within the CI workflow that would be nice to abstract away, see this discussion for an example:

crucible/.github/workflows/crux-llvm-build.yml

Lines 153 to 157 in 3bdafe4

	(cd crucible; with_ghc ghc -o $(setup_bin) $(setup_src) && ./$(setup_bin) clean)
	(cd crucible-llvm; with_ghc ghc -o $(setup_bin) $(setup_src) && ./$(setup_bin) clean)
	(cd crux; with_ghc ghc -o $(setup_bin) $(setup_src) && ./$(setup_bin) clean)
	(cd crux-llvm; with_ghc ghc -o $(setup_bin) $(setup_src) && ./$(setup_bin) clean)
	(cd uc-crux-llvm; with_ghc ghc -o $(setup_bin) $(setup_src) && ./$(setup_bin) clean)