cluster upgrade validation fails if eks-a kubeconfig namespace is not 'default'

**What happened**:

When using the same kubeconfig as the EKS-A tooling, running `eksctl anywhere upgrade cluster` with a namespace other than 'default' selected causes one of the checks to fail:

```
Error: failed to upgrade cluster: validations failed: getting eksa TinkerbellMachineConfig Error from server (NotFound): tinkerbellmachineconfigs.anywhere.eks.amazonaws.com "eks-a-cp" not found
```

It looks like one of the checks doesn't use `--namespace` in the kubectl command.

**What you expected to happen**:

The checks should pass regardless of the current namespace.

**How to reproduce it (as minimally and precisely as possible)**:

This is easy to reproduce:

```bash
# use the same kubeconfig as EKS-A tooling
export KUBECONFIG=eks-a/eks-a-eks-a-cluster.kubeconfig

$ kubectl get nodes
NAME STATUS ROLES AGE VERSION
cplane-0 Ready control-plane 35m v1.31.9-eks-ca3410b
cplane-1 Ready control-plane 31m v1.31.9-eks-ca3410b
cplane-2 Ready control-plane 25m v1.31.9-eks-ca3410b
worker-0 Ready 17m v1.31.9-eks-ca3410b
worker-1 Ready 17m v1.31.9-eks-ca3410b
```

```bash
# switch the namespace to something else than default
$ kubens -c
default

$ kubens eksa-system
✔ Active namespace is "eksa-system"

$ kubens -c
eksa-system
```

```bash
# start an `upgrade cluster` without changing any settings in `cluster.yaml`
eksctl anywhere upgrade cluster --hardware-csv hardware.csv -f cluster.yaml --no-timeouts
```

Performing setup and validations
...
✅ Upgrade cluster worker node group kubernetes version increment
✅ Validate authentication for git provider
❌ Validation failed {"validation": "validate immutable fields", "error": "getting eksa TinkerbellMachineConfig Error from server (NotFound): tinkerbellmachineconfigs.anywhere.eks.amazonaws.com \"eks-a-cp\" not found\n", "remediation": ""}
✅ Validate cluster's eksaVersion matches EKS-Anywhere Version
✅ Validate eksa controller is not paused

---

This simple fix would be helpful if you want to build an automation around `eksctl anywhere`.

---

**EKS Anywhere Release**
```
$ eksctl anywhere version
Version: v0.22.6
Release Manifest URL: https://anywhere-assets.eks.amazonaws.com/releases/eks-a/manifest.yaml
Bundle Manifest URL: https://anywhere-assets.eks.amazonaws.com/releases/bundles/100/manifest.yaml
```

Reproduce the failure with `eksctl anywhere upgrade cluster` while the kubeconfig namespace is `eksa-system`, then trace the immutable-fields validation and the kubectl command it invokes. Done means the upgrade validations pass with a non-default current namespace; the issue names no source files or tests to start from.

Written by the indexing model from the issue text.