Build cache should consider changes to the compiler

[celinval]

Proposed change: For internal development, we should include information about the current compiler version (e.g.: the commit hash or a hash of the kani-compiler binary. We should also include the toolchain version that's being used.

Motivation: This will alleviate the development pain of remembering to do a cargo clean whenever making changes to the compiler. Not doing so can manifest in weird / inconsistent results that end up taking time to debug.

[celinval]

Some context: rust-lang/cargo#8073

[celinval]

For Kani developers, if you make any change to kani-compiler or the rust toolchain, you might need to clean the build cache. For the regression, remove the build/ folder in the repo root, for one off test, delete the target directly. Another option is to pass the --force-rebuild flag to cargo kani