ci(e2e-cluster): export KUBECONFIG before acceptance runs

y-cluster v0.3.3 errors out with "KUBECONFIG env must be set" at
provision time -- the binary refuses to default a path so it can
never accidentally write to a developer's main kubeconfig. The
e2e-cluster job's runner has no KUBECONFIG set by default.

Add a pre-step that exports `$HOME/.kube/yolean` via $GITHUB_ENV
(matching the ystack convention from the local dev workflow), and
mkdir the parent dir. The acceptance script picks it up via the env
inherited through the ENV_IS_CLEAN=true trampoline-skip path.

Also retire the qemu/kvm pre-flight checks now that the acceptance
runs against the docker provider; replace with `df -h` + `docker
info` for surfaced disk + docker daemon visibility.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Yolean k8s-qa

.github/workflows/checks.yaml

@@ -61,18 +61,21 @@ jobs:
     timeout-minutes: 30
     steps:
     - uses: actions/checkout@v5.0.0
-    - name: Pre-flight (KVM, disk, qemu)
+    - name: Pre-flight (disk, docker)
       run: |
-        ls -l /dev/kvm
         df -h /
-        qemu-system-x86_64 --version | head -1
+        docker info | head -10
+    - name: Set KUBECONFIG (ystack convention)
+      run: |
+        mkdir -p "$HOME/.kube"
+        echo "KUBECONFIG=$HOME/.kube/yolean" >> "$GITHUB_ENV"
     - name: Run cluster acceptance
       env:
         # The script's first action is `exec env -i ...` which wipes
         # the env to "mirror a fresh interactive terminal" on a dev
         # laptop. CI's environment is already minimal; setting
-        # ENV_IS_CLEAN=true skips the trampoline so PATH / YSTACK_HOME
-        # below take effect.
+        # ENV_IS_CLEAN=true skips the trampoline so KUBECONFIG /
+        # PATH / YSTACK_HOME below take effect.
         ENV_IS_CLEAN: "true"
         YSTACK_HOME: ${{ github.workspace }}
         PATH: ${{ github.workspace }}/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin