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/packer/metadata.yaml

@@ -1,8 +1,18 @@
+---
+arch: amd64
+builds:
+  - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
+    release: '1'
+    repository: [almalinux/el8]
+    version: 1.13.1
+  - image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+    release: '1'
+    repository: [almalinux/el9]
+    version: 1.13.1
+description: Create identical images for multiple platforms from a single source configuration.
+github: unknown/packer
+homepage: https://www.packer.io/
+license: BUSL-1.1
+maintainer: HashiCorp
 name: packer
-release: 1
-version: 1.13.1
-build:
-- distro: el/8
-  image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
-- distro: el/9
-  image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+platform: linux