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/incus/resources/nfpm_incus.yaml

@@ -92,7 +92,7 @@ contents:
 
 # Scripts to run during installation/removal (optional)
 scripts:
-  postinstall: ./scripts/postinstall.sh
-  preremove: ./scripts/preremove.sh
-  postremove: ./scripts/postremove.sh
-  preinstall: ./scripts/preinstall.sh
+  postinstall: ./resources/scripts/postinstall.sh
+  preremove: ./resources/scripts/preremove.sh
+  postremove: ./resources/scripts/postremove.sh
+  preinstall: ./resources/scripts/preinstall.sh