The change in e404c311 broke models that
relied on single-type inheritance:

```
CommitStatus.new(type: 'Ci::Build').class
=> Ci::Build

CommitStatus.new.tap{|c| c.assign_attributes(type: 'Ci::Build')}.class
=> CommitStatus
```

As a result, any serialized options would not be properly saved to the
database, leading to errors expecting a `Hash` vs. a `String`, for
example.

This affected all pipelines that used shared runners because the build
would error out due to this serialization error.

Closes https://gitlab.com/gitlab-org/gitlab/issues/34860