Support reading in registries-config via env

[padlar]

Fixes #1198

[padlar]

Output with registries.Config set - read from env

➜  k3d git:(expose-config-env) make build && K3D_REGISTRIES_CONFIG=test2.yaml K3D_OPTIONS_RUNTIME_HOSTPIDMODE=true /Users/padlar/sandbox/k3d/bin/k3d cluster create test-cluster$ --verbose
fatal: No names found, cannot describe anything.
CGO_ENABLED=0 go build -mod readonly -tags '' -ldflags '-w -s -X github.com/k3d-io/k3d/v5/version.Version=42f404c6 -X github.com/k3d-io/k3d/v5/version.K3sVersion=v1.25.4-k3s1' -gcflags '' -o '/Users/padlar/sandbox/k3d/bin/k3d'
DEBU[0000] DOCKER_SOCK=/var/run/docker.sock
DEBU[0000] Runtime Info:
&{Name:docker Endpoint:/var/run/docker.sock Version:20.10.12 OSType:linux OS:Docker Desktop Arch:x86_64 CgroupVersion:2 CgroupDriver:cgroupfs Filesystem:extfs}
DEBU[0000] Additional CLI Configuration:
cli:
  api-port: ""
  env: []
  k3s-node-labels: []
  k3sargs: []
  ports: []
  registries:
    create: ""
  runtime-labels: []
  volumes: []
hostaliases: []
DEBU[0000] Configuration:
agents: 0
image: docker.io/rancher/k3s:v1.25.4-k3s1
network: ""
options:
  k3d:
    disableimagevolume: false
    disableloadbalancer: false
    disablerollback: false
    loadbalancer:
      configoverrides: []
    timeout: 0s
    wait: true
  kubeconfig:
    switchcurrentcontext: true
    updatedefaultkubeconfig: true
  runtime:
    agentsmemory: ""
    gpurequest: ""
    hostpidmode: "true"
    serversmemory: ""
registries:
  config: test2.yaml
  use: []
servers: 1

[erikgb]

@iwilltry42 Do you mind taking a look at this (small) PR? 🙏

[erikgb]

LGTM

[iwilltry42]

Thank you!