Rust backend always includes runtime code, doesn't emit it unless building the target program

[robin-aws]

Two separate but related issues:

1. The runtime source should only be emitted if --include-runtime is true, but the Rust backend ignores this option and always emits it.
2. The code to emit the runtime source is in RustBackend.CompileTargetProgram, so it's only run if /compile is at least 1, or the command is dafny build or run or test. It should also be emitted even if /compile is 0, or the command is translate.