Add build.os to readthedocs.yml

[will-moore]

Fixes this build failure: https://readthedocs.org/projects/ngff/builds/22266168/ which refers to https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os

(seen on https://github.com/ome/ngff/pull/218)

[github-actions[bot]]

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[will-moore]

readthedocs failure above...

Error
Problem in your project's configuration. Invalid configuration option "build.tools": At least one item should be provided in 'tools' or 'commands'

[will-moore]

All green - good to merge?

[joshmoore]

Thanks, @will-moore!