Ensure manifest_path is read from config properly by Assembly

[keiththomps]

Ran into the following error attempting to utilize propshaft main (caused by simple typo):

Test likely isn't necessary to keep, but I needed it to reproduce the issue.

[tapster]

Just hit the same issue. Confirmed Keith's fix in this PR resolves 👍 . thanks

2.943 bin/rails aborted!
2.945 NoMethodError: undefined method `manifest_path' for an instance of Pathname (NoMethodError)
2.945
2.945 load_path: load_path, output_path: config.output_path, compilers: compilers, manifest_path: config.manifest_path.manifest_path
2.945                                                                                                                  ^^^^^^^^^^^^^^