Update actions/checkout action to v4

[renovate]

This PR contains the following updates:

Package	Type	Update	New value	References	Sourcegraph
actions/checkout	action	major	v4	source

Test plan: CI should pass with updated dependencies. No review required: this is an automated dependency update PR.

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Release Notes

actions/checkout (actions/checkout)

v4

Compare Source

• Bump the minor-npm-dependencies group across 1 directory with 4 updates by @​dependabot in https://github.com/actions/checkout/pull/1739
• Bump actions/checkout from 3 to 4 by @​dependabot in https://github.com/actions/checkout/pull/1697
• Check out other refs/* by commit by @​orhantoy in https://github.com/actions/checkout/pull/1774
• Pin actions/checkout's own workflows to a known, good, stable version. by @​jww3 in https://github.com/actions/checkout/pull/1776

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