Cache not used if key contains "/" in 18.2.0

Summary

After upgrading to the latest 18.2.0 release of the gitlab runner, builds started taking a long time. On investigation it appears the cache is not being used and logs like

Restoring cache
cache key "0_libs/common-validation/-ad2ce3cf603c8374f21a90059218d7ffba330f5b-79-protected" must not contain '/' or its URL-encoded equivalent

can be seen.

Steps to reproduce

Configure the cache key like

  cache:
    key:
      files:
        - libs/common-validation/pom.xml

Actual behavior

The cache doesn't download and is not used from S3

Expected behavior

The S3 cache should be used as it was previously

Relevant logs and/or screenshots

Restoring cache
cache key "0_libs/common-validation/-ad2ce3cf603c8374f21a90059218d7ffba330f5b-79-protected" must not contain '/' or its URL-encoded equivalent

Used GitLab Runner version

18.2.0

Possible fixes

Probably related to !5719 (merged)

Workaround

Set cache:key:prefix in addition to cache:key:files, which will make GitLab provide the runner with a valid cache key again.