CI: build SRPM in Copr as suggested by Packit docs

Author: lzaoral

.packit.yaml

@@ -13,6 +13,8 @@ upstream_package_name: cscppc
 # downstream (Fedora) RPM package name
 downstream_package_name: cscppc
 
+srpm_build_deps: [rpm-build]
+
 release_suffix: ""
 actions:
     post-upstream-clone: ./make-srpm.sh --generate-sources