fix(ci): guard release Clean Workspace against leaked iso-builder devtmpfs

The iso job mounts a kernel-global devtmpfs under the workspace
(.cache/rootfs/dev). A killed job skips its EXIT-trap unmount and leaks
the bind; the verify job (same ci-single runner) then ran a bare
'rm -rf $WORKSPACE/*' that could traverse it and wipe the host /dev.

Detach any mount under the workspace before deleting and add
--one-file-system so rm never crosses a mount boundary. Same backstop
added to the iso job's own cleanup.

.github/workflows/release.yml

@@ -131,7 +131,16 @@ jobs:
       REPO: ${{ github.repository }}
     steps:
       - name: Clean Workspace
-        run: rm -rf "$GITHUB_WORKSPACE"/*
+        run: |
+          # This runner (ci-single) is shared with the iso job, which mounts a
+          # kernel-global devtmpfs at <workspace>/mulga/scripts/iso-builder/.cache/rootfs/dev.
+          # If that job is killed its EXIT-trap unmount never runs and the bind
+          # leaks; a plain rm -rf then traverses it and deletes the host's /dev
+          # (wipes /dev/null, wedges the box). Detach any leaked mounts under the
+          # workspace first, then forbid rm from crossing a mount boundary.
+          awk -v w="$GITHUB_WORKSPACE" '$2 ~ "^"w {print $2}' /proc/mounts \
+            | sort -r | xargs -r -I{} sudo umount -l -R {} || true
+          sudo rm -rf --one-file-system "$GITHUB_WORKSPACE"/*
 
       - uses: actions/create-github-app-token@v3
         id: app-token
@@ -216,7 +225,7 @@ jobs:
           sudo umount -l "${ROOTFS}/dev" 2>/dev/null || true
           sudo umount -l "${ROOTFS}/sys" 2>/dev/null || true
           sudo umount -l "${ROOTFS}/proc" 2>/dev/null || true
-          sudo rm -rf "$GITHUB_WORKSPACE"/*
+          sudo rm -rf --one-file-system "$GITHUB_WORKSPACE"/*
 
       - uses: actions/create-github-app-token@v3
         id: app-token