Bug 1973336: fix building http3server when --with-system-nss/nspr is used r=glandium,necko-reviewers,jesup a=RyanVM

Differential Revision: https://phabricator.services.mozilla.com/D258821

netwerk/test/http3server/moz.build

@@ -15,4 +15,7 @@ if CONFIG["COMPILE_ENVIRONMENT"]:
 if CONFIG["OS_ARCH"] == "Darwin":
     LDFLAGS += ["-Wl,-rpath,@executable_path"]
 
-USE_LIBS += ["nss"]
+if CONFIG["MOZ_SYSTEM_NSS"]:
+    LDFLAGS += CONFIG["NSS_LIBS"]
+else:
+    USE_LIBS += ["nss"]