To parse the manifest, we have to write it out so our regular manifest
loading code could handle it. This updates the manifest parsing code to
handle it.
This doesn't mean this will work everywhere in all cases though. For
example, ephemeral workspaces parses a manifest from the SourceId and
these won't have valid SourceIds.
As a consequence, `Cargo.lock` and `CARGO_TARGET_DIR` are changing from being next to
the temp manifest to being next to the script. This still isn't the
desired behavior but stepping stones.
This also exposes the fact that we didn't disable `autobins` like the
documentation says we should.
