scripts: add auth for sccache on protected branches

Why

Protected branches likely require access to a separate, secure GCS bucket for storing the Rust compilation cache. Without proper authentication, sccache cannot write to or read from this protected storage backend, which would result in reduced cache effectiveness and slower CI builds on protected branches.

How

The implementation adds conditional authentication logic after sccache initialization:

• On protected branches (CI_COMMIT_REF_PROTECTED = true):

  • Decodes the GCP_PROTECTED_SERVICE_ACCOUNT secret (base64 encoded)
  • Activates the GCP service account using gcloud auth activate-service-account
  • Obtains an OAuth access token via gcloud auth print-access-token
  • Exports the token as GOOGLE_OAUTH_ACCESS_TOKEN for sccache to use

• On non-protected branches: No authentication is performed, maintaining the existing behavior

To test this MR, I protected this branch, then I run this pipeline: xxx

runs as protected account : https://gitlab.com/tezos/tezos/-/jobs/12440195653

you can check that we log in the protected service account, and we should then have access to the protected bucket. for example in this job : https://gitlab.com/tezos/tezos/-/jobs/12453443094

Fix: #8161 (closed)