ENH: Add release logic to 'update-schema'

[mferrera]

Resolves #907
Stacked on #957

This will cause the schemas to overwrite with an updated $id field, but
only if nothing else has changed. It is also tested in GitHub Actions.

See https://github.com/equinor/fmu-dataio/actions/runs/12671501765/job/35313394311?pr=960#step:6:13 for how this looks when trying to merge a non-release version of the schema into the staging branch.

When preparing a PR for the release branch, you just need to run SCHEMA_RELEASE=1 ./tools/update-schema

[mferrera]

Originally went with --release but this struggles with the environment variable oriented way the schema URLs are determined. In some ways it's a bit nicer to manage this with an environment variable, so the adjusted worklflow just sets the environment variable before running the update-schema tool.

[jcrivenaes]

👍