Unverified binary generation

When testing compiler modifications we generally don't want to wait for a verified binary to be compiled in-logic. This issue is about creating an unverified binary pipeline. The idea is to translate the PureCake compiler to CakeML in-logic, and print the result to S-expressions. Then we should be able to use a pre-built CakeML binary to produce an executable PureCake compiler.

Overall:
1. Create a folder `compiler/binary/unverified` which prints the result of `pure_compilerProgTheory` to S-expressions
2. Add a `Makefile` in this directory for downloading the latest CakeML and compiling the S-expressions to produce a PureCake executable (borrowing from `examples/Makefile`)
3. Update the GitHub Actions workflow for creating a binary (`.github/binary.yml`) to use the unverified pathway