[ci] [dune] Updates to dune builds artifacts.

Closes #10694

We modify handling of artifacts for dune builds:
- we preserve `_build/log` log as artifact for ci-builds,
- we use a tar.gz as to preserve file permissions which is necessary
  in order to reuse the artifacts in an incremental build.

Dune uses this rule to digest a file:

```
  Digest.generic (file_contents f, f.stats.st_perm land 0o100 (* Only take USR_X in account *))
```

Since a few releases, Gitlab CI uses `.zip` as the storage format for
artifacts, this means that files in `_build` will get the executable
bit set when they did not have it originally, making all the digest
cache to miss. This caused #10694 .

See https://gitlab.com/gitlab-org/gitlab/issues/18711

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