ci: consume shared edera-dev/actions for disk and sccache steps

[tycho]

The disk-reporting, disk-reclaim, and sccache-credential-selection steps that grew here over the recent disk-pressure and compile-cache work have been extracted into reusable composite actions in edera-dev/actions (v0.0.15), so xen-oci and later builders share one implementation instead of drifting copies. Replace the inline steps with the actions, pinned by commit hash.

Behavior is intentionally identical, with two structural changes:

• The separate "report disk space (after cleanup)" step is gone; the reclaim action prints the KB it freed and the resulting disk state itself.
• The Azure connection string no longer travels via GITHUB_ENV (which lands in the process environment of every subsequent step, including third-party actions); the configure action emits it as a step output and only the "run docker script" step binds it as env, which is the single place docker.sh's -e passthrough needs it. The non-secret SCCACHE_AZURE_{RW_MODE,BLOB_CONTAINER,KEY_PREFIX} still arrive via GITHUB_ENV from the action. Fork PRs (no secrets) still degrade to a local disk cache: the container name is only exported when credentials exist, and sccache's Azure backend engages only when both are present.

The reclaim action's dangerous removals (hosted toolcache, Rust) are opt-in defaults-off in the shared action; this workflow opts into both, since kernel builds use neither and cosign-installer runs after the reclaim and re-creates its toolcache entry.