Purge the whole cache, report the cache tags not written - #431

Summary

Fixes the purge of the layers cache, and reports the cache tags that a run did not write.

• Bug
• Feature
• Breaking changes

• The purge listed the versions of the cache page by page while deleting them: each deletion shifted the next pages,
  so some versions were never listed nor deleted (one remained after the first purge of thecodingmachine/php-cache).
  It now reads the whole list first, then deletes it, and prints the number of versions deleted.
• The two cold runs of v5 this morning did not write 6 and 9 cache tags of slim images, without any error in the
  job: the cache export ignores its errors and the build was quiet. The build now writes its full output in
  build.log, kept 7 days as an artifact of the job, and the runs of v5 show a warning listing the cache tags not
  written. The published images were not affected: the next run rebuilds these targets and writes their cache.

The build log holds the output of the build steps, as the run of a pull request: no secret is passed to the build,
the registry logins only appear as [auth] … token for ghcr.io steps, without the token.

Test plan (required)

• Purge on a fork with pages of 10 versions: the old purge listed 131 of the 140 versions and left the 9 others,
  the new one deleted all of them. On this repository, it deleted the 202 versions of the cache.
• 4 cold runs since this morning (3 on a test branch of this repository with the build log, 1 of v5) wrote the
  161 cache tags: the missing tags were not reproduced.
• The check of the cache tags was run against the cache of this repository (no tag missing), and with a fake
  expected tag (reported).

Checklist

• I followed the guidelines in CONTRIBUTING guide
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code