gen_dwarf: run container as root, resolve deps via dnf, fix holder PID

[taodd]

The refresh-embedded-dwarf workflow's manual dispatch failed for every version. Four issues in gen_dwarf_for_version.sh, fixed here:

• Drop --userns=keep-id: it mapped the container process to the host's non-root UID, so dnf/rpm failed with "must be run as superuser". The rootless-podman default already maps container root back to the host user, so bind-mounted output stays host-owned.
• Add --allowerasing to the build-deps install so dnf can swap the image's curl-minimal for the requested curl (they conflict).
• Replace 'rpm -ivh --force' with a dnf install against the pinned per-release ceph repo + EPEL, so ceph-osd's full runtime dependency closure (ceph-base, libaio, thrift, tcmalloc, python3-rados, ...) is installed -- otherwise ceph-osd can't reach its entry point under gdb.
• Locate the held ceph-osd via gdb's PID, not the gdb 'shell' command's PID: ceph-osd and the keep-alive shell are siblings (both children of gdb), so the old 'pgrep -P holder' never matched.

Validated end-to-end locally for 19.2.2: both JSONs generated and the embedded-header re-aggregate + relink passes.