Adding option to select different `brane-chk` backends

[Lut99]

The new refactor is nearly complete. One of the only things missing is the capability to compile brane-chk for multiple backends instead of just the Haskell one.

Aside from changes in the interface code to call the correct one, a scheme for properly compiling either the Go-or the Haskell eFLINT-binaries must be concocted. Specifically, this is now done as a separate build target in Dockerfile.rls; however, this may be hard to do conditionally. As such, this building may need to be moved to a build.rs instead.