relative paths does not work in Infra.toml for signing_keys

[mikn]

Here you call this function: https://github.com/bottlerocket-os/twoliter/blob/develop/tools/pubsys-config/src/lib.rs#L185C53-L185C67

Which according to docs;
https://docs.rs/url/latest/url/struct.Url.html#method.from_file_path

requires the path to be absolute. If you try to create an Infra.toml that refers to a key relative to it, (as you have an example for in the Infra.toml.example file) it will fail with the relatively unintuitive error: Unable to build URL from signing key for repo 'default'

The error could be improved by passing on the error from the inner error, and also perhaps the actual parameter that caused the error. I am unsure how you could handle this without not using the url crate as it doesn't seem to carry any functions to allow for relative paths - but I am also unsure why you would need to support an URI-style path to the file here?