node_repositories in WORKSPACE missing linux_ppc64le and linux_s390x entries for Node.js 18.20.8

Body:

## Summary

TensorBoard 2.21.0 manually specifies Node.js 18.20.8 binary SHAs via `node_repositories()` in the WORKSPACE file, but only includes entries for five platforms: `darwin_arm64`, `darwin_amd64`, `linux_arm64`, `linux_amd64`, and `windows_amd64`.

The `linux_ppc64le` and `linux_s390x` entries are missing, even though Node.js 18.20.8 publishes official binaries for both at https://nodejs.org/dist/v18.20.8/.

## Error

Building TensorBoard 2.21.0 from source on ppc64le (or s390x) with Bazel fails:

ERROR: No nodejs is available for linux_ppc64le at version 18.20.8

## Suggested Fix

Add the missing entries to `node_repositories` in the WORKSPACE:

```python
"18.20.8-linux_ppc64le": ("node-v18.20.8-linux-ppc64le.tar.xz", "node-v18.20.8-linux-ppc64le", "57fde85b5f8e6b3c5d45e48a82dbd28f1d2dde816b0da2b5fc250c1a63ea49e6"),
"18.20.8-linux_s390x": ("node-v18.20.8-linux-s390x.tar.xz", "node-v18.20.8-linux-s390x", "35c81e08d26e25598e47dc4d1755f6bc0be802457a2f5bf775ead2ffccb73e89"),

SHA256 checksums from official SHASUMS256.txt.

Context

• rules_nodejs added ppc64le/s390x support in bazelbuild/rules_nodejs#3574 (Oct 2022)
• TensorBoard 2.20.0 used rules_nodejs 5.7.x which had built-in Node.js tables including these platforms
• TensorBoard 2.21.0 switched to 5.8.1 with a manual node_repositories dict that inadvertently dropped ppc64le/s390x

Environment

• TensorBoard: 2.21.0
• Bazel: 7.6.1
• Platform: linux_ppc64le (also affects linux_s390x)
• rules_nodejs: 5.8.1

Open the WORKSPACE file and inspect the existing node_repositories entries for Node.js 18.20.8, then verify the provided filenames, prefixes, and SHA256 values against the official SHASUMS256.txt. Done means linux_ppc64le and linux_s390x resolve under rules_nodejs 5.8.1 and the reported Bazel platform error is no longer produced.