Rename custom workflows to prevent conflicts with ci-mgmt (#1483)

Preparation for pulumi/ci-mgmt#1142.

This renames custom workflows to have an "eks-" prefix to prevent
ci-mgmt from deleting them (similar to
pulumi/pulumi-aws#4418).

This also adds a missing `.actionlint.yml` config file, copied from
[azure-native](https://github.com/pulumi/pulumi-azure-native/blob/master/.actionlint.yml).
The `lint_workflows` action currently has no effect without this file
([example](https://github.com/pulumi/pulumi-eks/actions/runs/11468567327/job/31913961281)):

> could not read config file ".actionlint.yml": open .actionlint.yml: no
such file or directory

.actionlint.yml

@@ -0,0 +1,4 @@
+self-hosted-runner:
+  # Labels of self-hosted runner in array of string
+  labels:
+    - pulumi-ubuntu-8core