bin: install kubelogin as the kubectl oidc-login plugin

omnictl-generated kubeconfigs authenticate via `kubectl oidc-login
get-token`, so install int128/kubelogin as kubectl-oidc_login.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -41,7 +41,8 @@ Already have a clone? Just run it directly:
   Docker daemon is already running (e.g. Kasm) so it never fights that
   setup. Uses sudo.
 - `bin/` — installs userland devops CLIs as self-contained, statically
-  linked release binaries into `~/.local/bin`: jq, kubectl, helm, k9s,
+  linked release binaries into `~/.local/bin`: jq, kubectl, kubelogin (the
+  `kubectl oidc-login` plugin, for OIDC auth to Omni clusters), helm, k9s,
   kubectx/kubens, stern, yq, OpenTofu, terragrunt, task (go-task, the
   Taskfile runner), the 1Password CLI `op` for secrets storage, the
   opencode coding agent, zarf (DevSecOps for Offline and Airgapped