Always pass in -Zno-codegen

[atomb]

Currently, if you run cargo mir-json on an unbuilt repository, you'll get a .mir file out, as well as target binaries. If you run it on an already-built repository (even if built with cargo build instead of cargo mir-json) then the JSON translation doesn't happen. If we consistently pass the -Zno-trans flag throughout, we'll avoid the time taken to build native code as well as the problem of re-running the translation.

[RyanGlScott]

Note that -Zno-trans is now called -Zno-codegen after rust-lang/rust#50615.