GitHub Actions: update to checkout@v4

We seem to be getting "Node.js 16 actions are deprecated." warnings
for jobs that use checkout@v3.  Except for the i686 containers job
that is kept at checkout@v1 [*], update to checkout@v4, which is
said to use Node.js 20.

[*] 6cf4d908 (ci(main): upgrade actions/checkout to v3, 2022-12-05)
    refers to https://github.com/actions/runner/issues/2115 and
    explains why container jobs are kept at checkout@v1.  We may
    want to check the current status of the issue and move it to the
    same version as other jobs, but that is outside the scope of
    this step.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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