refactor: modernise RPM builder with Python tooling v2

- Migrate from legacy shell-based build system to modern Python tooling
- Update all metadata.yaml files to new schema with per-distro builds
- Standardise build scripts with curl -L, envsubst, and error handling
- Convert nfpm.yaml templates to use environment variable substitution
- Update Dockerfile to accept all package metadata as build arguments
- Modernise Makefile to use new Python build tool commands
- Update CI workflow to use tools/build instead of make

rpms/helmfile/metadata.yaml

@@ -1,9 +1,18 @@
+---
+arch: amd64
+builds:
+  - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
+    release: '1'
+    repository: [almalinux/el8]
+    version: 1.1.7
+  - image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+    release: '1'
+    repository: [almalinux/el9]
+    version: 1.1.7
+description: A declarative spec for deploying Helm charts. It lets you keep a directory of chart value files and maintain changes in version control; apply CI/CD to configuration changes; and periodically sync to avoid skew in environments.
+github: helmfile/helmfile
+homepage: https://github.com/helmfile/helmfile
+license: MIT
+maintainer: Helmfile Contributors
 name: helmfile
-release: 1
-version: 1.1.7
-build:
-- distro: el/8
-  image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
-- distro: el/9
-  image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
-github: helmfile/helmfile
+platform: linux