Fix missing environment variable declaration for nginx.

Things seemed to work fine without this declaration, except when
experimenting with disabling Lua's code cache in development. Then this
environment variable was no longer available via os.getenv after the
first request load.

templates/etc/nginx/router.conf.mustache

@@ -14,6 +14,7 @@ events {
 
 error_log stderr;
 
+env API_UMBRELLA_SRC_ROOT;
 env API_UMBRELLA_RUNTIME_CONFIG;
 
 pcre_jit on;