use mako's TemplateLookup rather than custom code

Related to race-condition observed w/ python3.11, see:

sqlalchemy/mako#378

This will likely not fully solve race-conditions, as those also occur
between rendering from passed string and from file if called
concurrently.

However, using lookup from mako consistently is closer to how Mako is
intended to be used + there seems to be synchronisation between
lookup-calls at least, which should reduce the frequency of concurrency
issues.

concourse/steps/__init__.py

@@ -16,17 +16,14 @@
 import os
 
 import mako.template
-
-import ci.util
-
+import mako.lookup
 
 steps_dir = os.path.abspath(os.path.dirname(__file__))
+template_lookup = mako.lookup.TemplateLookup(directories=(steps_dir,))
 
 
 def step_template(name):
-    step_file = ci.util.existing_file(os.path.join(steps_dir, name + '.mako'))
-
-    return mako.template.Template(filename=step_file)
+    return template_lookup.get_template(f'/{name}.mako')
 
 
 def step_def(name):