`yarn install` doesn't install enough things to make `yarn lint` work

The `flake8` and `lint-python` commands fail when I run `yarn install && yarn lint` on a fresh clone:

```
% yarn lint
yarn run v1.22.22
$ run-p lint-js lint-python prettier-run
$ prettier --check .
$ flake8 tools loadtest
$ eslint src bin test
/bin/sh: flake8: command not found
error Command failed with exit code 127.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
ERROR: "lint-python" exited with 127.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this c
```

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-986)