ci: hard-code ubuntu-22.04 for Python 3.7 CI jobs

ubuntu-latest is in the process of being updated from ubuntu-22.04 to
ubuntu-24.04, which leads to issues with py37:

```
Version 3.7 was not found in the local cache
Error: The version '3.7' with architecture 'x64' was not found for Ubuntu 24.04.
```

This commit hard-codes the ubuntu-version for py37 to ubuntu-22.04,
similar to what we've done for py36.

See https://github.com/actions/runner-images/issues/10636 for more
information.