ci(pages): fix JupyterLite "No Kernel" — absolute argv[0] in xlean kernel.json

[junjihashimoto]

Problem

The hosted lab https://verilean.github.io/sparkle/lab/ shows "No Kernel" — no Lean 4 kernel is offered. The deployed xeus/kernels.json is [].

Closes #85.

Root cause

The WASM kernel rebuild step runs cmake --install into the staging prefix, which installs xeus-lean's source kernel.json. That template's argv is the bare relative string:

"argv": [ "xlean" ]

(The stock env kernel.json uses an absolute /opt/.../bin/xlean, but cmake --install clobbers it.)

jupyter lite build → jupyterlite_xeus.add_on.get_kernel_binaries reads argv[0] and checks argv[0] + ".js" / + ".wasm". With argv[0] == "xlean" those resolve relative to the build cwd (/work), where the binaries do not exist — the real ones are at $CAND/bin/xlean.{js,wasm}. So it warns "kernel binaries not found for xlean", drops the kernel, and kernels.json comes out [].

Fix

After cmake --install, rewrite the installed kernel.json's sole argv line from the relative "xlean" to the absolute $CAND/bin/xlean. Uses sed on the standalone argv line — the docs-builder image has no jq, and sed avoids the nested-heredoc quoting hazard inside the docker bash -c '…'.

Verification

Confirmed inside the docs-builder image that after the rewrite get_kernel_binaries resolves both xlean.js and xlean.wasm (FOUND, vs NOT FOUND before) — which is what populates xeus/kernels.json and lets the lab offer the Lean 4 kernel.