Add runner rule (#6)

* Add runner rule

* Run black against lint.py

lint-workflow/lint.py

@@ -25,6 +25,13 @@ def lint(filename):
             for job_key in jobs:
                 job = jobs[job_key]
 
+                # Make sure runner is using pinned version.
+                runner = job["runs-on"]
+                if runner.split("-")[1] == "latest":
+                    findings.append(
+                        f"- Runner version is set to '{runner}', but needs to be pinned to a version."
+                    )
+
                 # Check for 'name' key for job.
                 if "name" not in job:
                     findings.append(f"- Name key missing for job key '{job_key}'.")