Prevents stale artifacts from sticking around by accident!

The current manifests encode this with a dash in the name, so we should preserve
that!

Right now we just run `shasum` on an absolute path but right now the shasum
files only include filenames, so let's use `current_dir` and just the file name
to only have the file name emitted.

The previous fix contained an error where `toml::encode` returned a runtime
error, so this version just constructs a literal `toml::Value`.