Fix Azure cache not working in K8S executor

In the K8S executor, the object name is prefaced with the cache directory (e.g. /cache), which is then used to sign the object name. However, the cache archiver discards leading slashes when uploading the file, so Azure rejects the upload with a signature mismatch error.

To fix this, we normalize object names by dropping leading slashes.

Closes #27324 (closed)