chore(eks): set AWS_STS_REGIONAL_ENDPOINTS to regional for KubectlFunction (#30580)

### Issue # (if applicable)

Per boto/botocore#2577 (comment) setting this to `regional` is still required and recommended for boto3.

Closes #30496


### Reason for this change



### Description of changes



### Description of how you validated changes

Added a new unit test and verified with debugger.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

### Debugger

```json
{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "Jest",
      "program": "${workspaceFolder}/node_modules/jest/bin/jest.js",
      "cwd": "${workspaceFolder}/packages/aws-cdk-lib",
      "args": [
        "--verbose",
        "-i",
        "--no-cache",
        "test/kubectl-provider.test.ts",
      ],
      "console": "integratedTerminal",
      "internalConsoleOptions": "neverOpen",
      "skipFiles": [
        "<node_internals>/**"
      ],
      "outFiles": [
        "${workspaceFolder}/**/*.(m|c|)js",
        "!**/node_modules/**"
      ],
    }
  ]
}
```
----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: pahud

packages/@aws-cdk-testing/framework-integ/test/aws-eks/test/integ.alb-controller.js.snapshot/asset.e360285e7bd78b792c3d120b19dff9840b2c1a235eba49ce3ab0105470358b6f/index.js renamed to packages/@aws-cdk-testing/framework-integ/test/aws-eks/test/integ.alb-controller.js.snapshot/asset.6b19297a97597a250e5e30435d2cc30a5558a154c4135bccb767b62181e6c86d/index.js

@@ -1056,7 +1056,7 @@
        {
         "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
        },
-       "/12013bd7d8559f6883e853e9624275cf3f32d86c166243ab85c46269c009b249.json"
+       "/8cef919709334806828263085b34beb96a356e0826668c03914f8899bbe0f04e.json"
       ]
      ]
     }
@@ -1106,7 +1106,7 @@
        {
         "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
        },
-       "/9b57e71b3fd1cfcfdec66d3b197f9800f5ef70b999bb1e7b163e427671ecea0e.json"
+       "/6746b754b3db6762bc833cdc322e572da19c6b8c354197887eeab99e82edfe1c.json"
       ]
      ]
     }

packages/@aws-cdk-testing/framework-integ/test/aws-eks/test/integ.alb-controller.js.snapshot/aws-cdk-eks-cluster-alb-controller.assets.json

@@ -50,7 +50,7 @@
      "S3Bucket": {
       "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
      },
-     "S3Key": "e360285e7bd78b792c3d120b19dff9840b2c1a235eba49ce3ab0105470358b6f.zip"
+     "S3Key": "6b19297a97597a250e5e30435d2cc30a5558a154c4135bccb767b62181e6c86d.zip"
     },
     "Description": "onEvent handler for EKS cluster resource provider",
     "Environment": {
@@ -123,7 +123,7 @@
      "S3Bucket": {
       "Fn::Sub": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}"
      },
-     "S3Key": "e360285e7bd78b792c3d120b19dff9840b2c1a235eba49ce3ab0105470358b6f.zip"
+     "S3Key": "6b19297a97597a250e5e30435d2cc30a5558a154c4135bccb767b62181e6c86d.zip"
     },
     "Description": "isComplete handler for EKS cluster resource provider",
     "Environment": {

packages/@aws-cdk-testing/framework-integ/test/aws-eks/test/integ.alb-controller.js.snapshot/aws-cdk-eks-cluster-alb-controller.template.json

@@ -10,6 +10,11 @@
      "S3Key": "abc70c90ded969d12235ca11768293cb20557cff54518518480c0d9fb344a098.zip"
     },
     "Description": "onEvent handler for EKS kubectl resource provider",
+    "Environment": {
+     "Variables": {
+      "AWS_STS_REGIONAL_ENDPOINTS": "regional"
+     }
+    },
     "Handler": "index.handler",
     "Layers": [
      {