Fix: Add entry to schema/go.sum

[sudo-bmitch]

This is following up on #1253 (review)

This is the result of running:

cd schema && go get github.com/opencontainers/image-spec@main

Which allows commands in schema to run without a go.work with a fixed version of the image-spec.

[tianon]

I don't love it, but it seems fine.

[tianon]

We could also consider committing the go.work file: https://go.dev/ref/mod#:~:text=It%20is%20generally,and%20released%20properly.

That said, there are some cases where committing a go.work file makes sense. For example, when the modules in a repository are developed exclusively with each other but not together with external modules, there may not be a reason the developer would want to use a different combination of modules in a workspace. In that case, the module author should ensure the individual modules are tested and released properly.

[sudo-bmitch]

We could also consider committing the go.work file

I'd consider it if we controlled all the workflows where the repo is used. But since we don't, the reasons not to checkin the file feel like they apply to us.