Replace pre-commit.ci with a simple action that runs `prek`.

[samcunliffe]

@pre-commit.ci seems like it's having a mare

error: Request failed after 3 retries in 6.8s
  Caused by: Failed to fetch: ``https://pypi.org/simple/pytest-venv/``
  Caused by: error sending request for url (https://pypi.org/simple/pytest-venv/)
  Caused by: client error (Connect)
  Caused by: dns error
  Caused by: failed to lookup address information: Temporary failure in name resolution

Just realised our very own @dstansby made an issue on this pre-commit-ci/issues#249

Originally posted by @paddyroddy in #630 (comment)

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From the thread in pre-commit-ci/#249 it seems this is not going to be fixed (nor even documented).

It might be nice to anyway switch out pre-commit.ci to a simple actions workflow? Any opinions @UCL-ARC/collaborations-python-tooling ?