fix/chore(*): metadata label key updates, inject_env fix

[vdice]

• Update inject_envs_with_prefix to exit gracefully if no env vars are found (grep by default exits non-zero if it doesn't find a match)
  • I hit this when running with OUTPUT=none... previously the function would exit non-zero trying to find K6_ prefixed env vars when there were none
• Update the metadata label keys used by export_node_info (remove deprecations per https://kubernetes.io/docs/reference/labels-annotations-taints/)
• Update kind-config to add the instance-type label, or else it returns a weird null string (--tag node_instance_type=^[[0;90mnull^[[0m) which in turn prevents the test run from ever proceeding past the initializing stage

[kate-goldenring]

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