feat: add sigil templating to CHECKS files

This enables general templating capabilities, in addition to environment variable access.

Closes #3974.

tests/apps/nodejs-express/web.js

@@ -6,6 +6,10 @@ app.get('/', function(request, response) {
   response.send('nodejs/express');
 });
 
+app.get('/healthcheck', function(request, response) {
+  response.send(process.env.HEALTHCHECK_ENDPOINT);
+});
+
 var port = process.env.PORT || 5000;
 app.listen(port, function() {
   console.log("Listening on " + port);