bwc: pin podman auth to a persistent authfile

bwc: pin podman auth to a persistent authfile

podman login was writing credentials to the default location under
XDG_RUNTIME_DIR (/run/user/<uid>), which is logind-managed tmpfs. The
Jenkins agent runs without XDG_RUNTIME_DIR set, so the runtime dir can
be torn down or re-resolved between bwc_login and the podman build it
is meant to authenticate. When that happens, the base image pull inside
build-with-container.py silently falls back to anonymous and hits
Docker Hub's per-IP unauthenticated rate limit (toomanyrequests), e.g.:

  https://jenkins.ceph.com/job/ceph-pull-requests-arm64/99076/

Set REGISTRY_AUTH_FILE to a persistent path in $HOME and pass
--authfile to podman login so login and all subsequent podman/buildah
invocations in the job resolve the same credential file regardless of
session state.

Fixes: https://tracker.ceph.com/issues/77920
Signed-off-by: David Galloway <david.galloway@ibm.com>