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>