AGENTS.md: Add note about using failfast (#58696)

Author: IanButterworth

AGENTS.md

@@ -50,6 +50,7 @@ corresponding test to ensure that the test is still passing with your changes.
 - If you are adding a new test, add it to an existing test file. Do not create a new test file unless explicitly instructed.
 - Write one comment at the top of the test to explain what is being tested.
   Otherwise keep comments minimal.
+- Use the environment variable `JULIA_TEST_FAILFAST=1` to make tests fail fast.
 
 ### External dependencies