chore: update actions/checkout in lake new template

[kt3k]

This PR updates actions/checkout action to the latest in lake new / lake init templates.

ref https://github.com/actions/checkout

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase c8e19cca2547a89c3aba401afaab9a8808f490a2 --onto 138ca9f20763523c4093baa092cf371e89535098. You can force Mathlib CI using the force-mathlib-ci label. (2026-09-01 01:12:16)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase c8e19cca2547a89c3aba401afaab9a8808f490a2 --onto e991a05e359a25988f49bff3ab8af986e959b866. You can force reference manual CI using the force-manual-ci label. (2026-09-01 01:12:18)