CI: run pre-commit hooks in CI

This also means flake8 will output to stdout in CI now. It didn't
before because it output to an XML format for publishing.

Pre-commit creates its own environment in which it installs hooks. To
speed up runs, the pipeline will cache this for use with future jobs.
The cache will update if .pre-commit-config.yaml changes.

The flake8 pre-commit hook invokes flake8 via `pipenv run flake8`. It's
normally useful to use pipenv here cause it ensures flake8 is invoked
within the context of the venv. However, in CI, there is no venv -
dependencies are installed directly to the system site-packages.
`pipenv run` does not work in such case because it tries to create a new
venv if one doesn't exist (it doesn't consider the system interpreter to
be a venv).

This workaround (okay, it's a hack) creates an executable shell script
which replaces the original pipenv binary. The shell script simply
ignores the first argument (i.e. ignores `run` in `pipenv run`) and
executes the rest of the arguments as a command. It essentially makes
`pipenv run flake8` equivalent to just having ran `flake8`. When
pre-commit executes pipenv, the aforementioned script is what will run.

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