Look in more places for rust-toolchain and viper_tools

[zgrannan]

When using a local version of Prusti, and setting the "Local Prusti Path" to the prusti executable generated using ./x.py build in prusti-dev (i.e. prusti-dev/target/debug), the extension throws an error beause it cannot find rust-toolchain and viper_tools. This is because those files are in the folder prusti-dev, rather than prusti-dev/target/debug.

This PR resolves the issue by also looking in the parent directories of "Local Prusti Path" to locate viper_tools and rust-toolchain.

[fpoli]

Thanks! Feel free to merge. Originally the "Prusti local path" was meant to be the result of ./x.py package, but I agree that during development it's not convenient. If you want to trigger a new release of the extension on the marketplace, bump up the version in package.json.