pref: maintain order of 'debug' in compile defines as mod cache key depends on it

vlib/v/pref/default.v

@@ -92,11 +92,16 @@ pub fn (mut p Preferences) fill_with_defaults() {
 		} else {
 			// enable the GC by default
 			p.gc_mode = .boehm_full_opt
+			// NOTE: these are added to p.compile_defines[_all]
+			// more than once when building modules for usecache
 			p.parse_define('gcboehm')
 			p.parse_define('gcboehm_full')
 			p.parse_define('gcboehm_opt')
 		}
 	}
+	if p.is_debug {
+		p.parse_define('debug')
+	}
 	if p.os == ._auto {
 		// No OS specifed? Use current system
 		p.os = get_host_os()

vlib/v/pref/pref.v

@@ -733,9 +733,6 @@ pub fn parse_args_and_show_errors(known_external_commands []string, args []strin
 			}
 		}
 	}
-	if res.is_debug {
-		res.parse_define('debug')
-	}
 	if command == 'crun' {
 		res.is_crun = true
 	}