Set nullable=False on the ModuleBuild.context column model to match the migration
The context column was added in d83e6897ca8ac02da8186c5f2918107f8314a6e9. The
migration set `nullable=False`, but the model kept the default of `nullable=True`.
This caused `mbs-manager db migrate` to create a migration for setting the context
collumn to `nullable=True`.