install agent from the spaces registry

[avalanche123]

Description of your changes

Now that spaces images moved into xpkg registry, install agent from there too.

I have:

• Read and followed Upbound's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Created a space in kind and connected it, checked the images afterwards:

kubectl get pod -A -o json | jq '[.items[] | .spec.containers[] | .image] | sort | unique'
[
  "docker.io/kindest/kindnetd:v20240813-c6f155d6",
  "docker.io/kindest/local-path-provisioner:v20240813-c6f155d6",
  "quay.io/jetstack/cert-manager-cainjector:v1.11.0",
  "quay.io/jetstack/cert-manager-controller:v1.11.0",
  "quay.io/jetstack/cert-manager-webhook:v1.11.0",
  "registry.k8s.io/coredns/coredns:v1.11.1",
  "registry.k8s.io/etcd:3.5.15-0",
  "registry.k8s.io/ingress-nginx/controller:v1.8.1@sha256:e5c4824e7375fcf2a393e1c03c293b69759af37a9ca6abdb91b13d78a93da8bd",
  "registry.k8s.io/kube-apiserver:v1.31.0",
  "registry.k8s.io/kube-controller-manager:v1.31.0",
  "registry.k8s.io/kube-proxy:v1.31.0",
  "registry.k8s.io/kube-scheduler:v1.31.0",
  "xpkg.upbound.io/spaces-artifacts/agent:v0.0.0-429.g5433474",
  "xpkg.upbound.io/spaces-artifacts/envoy:v1.26-latest",
  "xpkg.upbound.io/spaces-artifacts/hyperspace:v1.7.0",
  "xpkg.upbound.io/spaces-artifacts/mxp-charts:v1.7.0"
]