Problems running checkmate tasks

### Problem

Checkmate is throwing an error when you attempt to run a task. We now have the tasks workflows running in GitHub Actions. A failed run can be found: [here](https://github.com/hypothesis/checkmate/runs/8155796167?check_suite_focus=true)

#### Error

```
Traceback (most recent call last):
File "/usr/local/bin/alembic", line 8, in
sys.exit(main())
File "/usr/local/lib/python3.8/site-packages/alembic/config.py", line 590, in main
CommandLine(prog=prog).main(argv=argv)
File "/usr/local/lib/python3.8/site-packages/alembic/config.py", line 584, in main
self.run_cmd(cfg, options)
File "/usr/local/lib/python3.8/site-packages/alembic/config.py", line 561, in run_cmd
fn(
File "/usr/local/lib/python3.8/site-packages/alembic/command.py", line 545, in current
script.run_env()
File "/usr/local/lib/python3.8/site-packages/alembic/script/base.py", line 569, in run_env
util.load_python_file(self.dir, "env.py")
File "/usr/local/lib/python3.8/site-packages/alembic/util/pyfiles.py", line 94, in load_python_file
module = load_module_py(module_id, path)
File "/usr/local/lib/python3.8/site-packages/alembic/util/pyfiles.py", line 110, in load_module_py
spec.loader.exec_module(module) # type: ignore
File "", line 843, in exec_module
File "", line 219, in _call_with_frames_removed
File "checkmate/migrations/env.py", line 11, in
from checkmate import models
ModuleNotFoundError: No module named 'checkmate'
```

The command being run is:

```
alembic -c conf/alembic.ini current
```