Mirrors the earlier release.yml switch to nfpm. The local `make hotfix`
flow now downloads nfpm v2.46.3 (matching CI), stages the binary at
dist/buckit, computes a numeric PKG_VERSION from the RELEASE tag with
the same transformation CI uses (and additionally strips the
.hotfix.<sha> suffix that hotfix-vars appends), and builds rpm/deb/apk
from packaging/nfpm.yaml. Resulting packages are copied into the
existing buckit-release/$(GOOS)-$(GOARCH)/ directory so hotfix-push
keeps working unchanged.
clean target updated to remove nfpm_*.deb and the new dist/ directory.
release-process-plan.md table row updated to reflect the tool change.
Note: no Go-module dependency on pkger ever existed; pkger was only a
CI/Make-time download.