yamllint: don't apply truthy test to map keys

yamllint complains about the GitHub workflow definition because of
the "on" map key, which it considers a "truthy" value.  Tweak the
yamllint config so that it no longer applies the "truthy" rule to
keys.

.yamllint.yaml

@@ -9,3 +9,5 @@ rules:
   line-length: disable
   #  max: 120
   comments-indentation: disable
+  truthy:
+    check-keys: false