Fix string to boolean conversion in bazelci.py (#1260)

* Fix string to boolean conversion in bazelci.py

I think the old code didn't actually work as intended - because os.environ can only contain strings, it didn't convert values like 'false' or '0' correctly.

* Address code review comments

* Update bazelci.py
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 25f96c3..1fb9eb7 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -969,12 +969,16 @@
 def print_expanded_group(name):
     eprint("\n\n+++ {0}\n\n".format(name))
 
+    
+def is_trueish(s):
+    return str(s).lower() in ["true", "1", "t", "y", "yes"]
+
 
 def use_bazelisk_migrate():
     """
     If USE_BAZELISK_MIGRATE is set, we use `bazelisk --migrate` to test incompatible flags.
     """
-    return bool(os.environ.get("USE_BAZELISK_MIGRATE"))
+    return is_trueish(os.environ.get("USE_BAZELISK_MIGRATE"))
 
 
 def bazelisk_flags():