Description of problem:
When `signers.x509.tuf.mirror.url` is configured to point to the TUF server which is provided by the Trusted Artifact Signer operator, 
the Tekton Chains logs the following error: "error configuring x509 signer: initialize tuf: creating cached local store: mkdir /.sigstore: read-only file system".

This occurs durring the x509 signer initialization and prevents Chains from using the TUF mirror.

Workaround: 
 
 A workaround have been to use default value, (https://sigstore-tuf-root.storage.googleapis.com)

Prerequisites (if any, like setup, operators/versions):

Red hat Openshift Pipelines version 1.20.0
Red Hat Trusted Artifact Signer version 1.2.1 
Red Hat Build of Keycloak Operator version 26.4.4-opr.1

Reproducibility (Always/Intermittent/Only Once):
Always