validate: yamllint: ignore "truthy value should be one of" warnings

Suppresses warnings like:

    LANG=C.UTF-8 yamllint -c hack/validate/yamllint.yaml -f parsable .github/workflows/*.yml
    .github/workflows/ci.yml:7:1: [warning] truthy value should be one of [false, true] (truthy)
    .github/workflows/windows.yml:7:1: [warning] truthy value should be one of [false, true] (truthy)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/validate/yamllint.yaml

@@ -3,3 +3,8 @@ locale: C.UTF-8
 rules:
   document-start: disable
   line-length: disable
+  # ignore "warning  truthy value should be one of [false, true]" on GitHub
+  # actions workflows, which use "on:" to specify when to run.
+  truthy:
+    ignore: |
+      .github/workflows/