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/etcd/resources/build.sh

@@ -1,9 +1,13 @@
 #!/usr/bin/bash
+set -e
 
 # Download and extract etcd
 wget -O /app/etcd-v${PACKAGE_VERSION}-linux-amd64.tar.gz https://github.com/etcd-io/etcd/releases/download/v${PACKAGE_VERSION}/etcd-v${PACKAGE_VERSION}-linux-amd64.tar.gz
 tar xf /app/etcd-v${PACKAGE_VERSION}-linux-amd64.tar.gz
 mv /app/etcd-v${PACKAGE_VERSION}-linux-amd64/* /app/
 
+# Process nfpm.yaml with envsubst
+envsubst < /app/resources/nfpm.yaml > /app/nfpm.yaml
+
 # Build the RPM
-nfpm pkg --config /app/resources/nfpm.yaml --target /app/dist --packager rpm
+nfpm pkg --config /app/nfpm.yaml --target /app/dist --packager rpm