ci: use v4 built-in checkout (builtin:checkout), not actions/checkout@v4

actions/checkout@v4 is a Node action; the runner downloads it and execs it
with `node`, which the slim rust image lacks (exit 127), and the action-copy
also trips a podman path quirk. `builtin:checkout` is gitea-runner v4's native
Go checkout: no download, no Node, no action copy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -14,7 +14,8 @@ jobs:
       image: docker.io/library/rust:1-bookworm
     steps:
       - name: Hent koden
-        uses: actions/checkout@v4
+        # v4 sin innebygde Go-checkout: ingen nedlasting, ingen Node i avbildet.
+        uses: builtin:checkout
 
       - name: Verktøykjede-komponenter
         # rust-avbildet er slankt og har ikke rustfmt/clippy med; legg dem til