Make the S3 test more robust (backport #13257)

[mergify]

Motivation

Waiting for the minio unit is apparently not reliable enough (see for example https://github.com/DeterminateSystems/nix-src/actions/runs/15220608881/job/42815546769), so let's also wait for the port.

Context

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This is an automatic backport of pull request #13257 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of ca96967 has failed:

On branch mergify/bp/2.28-maintenance/pr-13257
Your branch is up to date with 'origin/2.28-maintenance'.

You are currently cherry-picking commit ca9696748.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
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	both modified:   tests/nixos/s3-binary-cache-store.nix

no changes added to commit (use "git add" and/or "git commit -a")

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