feat: usmrad sets USM_VERBOSE so usm and build scripts stream their output

Failures in dependency installs and manifest builds now surface verbatim
in the container log.

src/rad/Usmrad.vala

@@ -208,6 +208,10 @@ public static int main(string[] args) {
         return 255;
     }
 
+    // Every spawned usm (and its nested build scripts) streams management
+    // script output, so failures surface in the container log verbatim
+    Environment.set_variable("USM_VERBOSE", "1", true);
+
     rad_source = new Source(rad_config.repo, rad_config.branch, rad_slots.repo_dir);
     rad_system = new System(rad_config, usm_binary);
     rad_deployer = new Deployer(rad_config, rad_slots, rad_source, rad_system, usm_binary);