[R-package] [ci] newer versions of actions/setup-pandoc are incompatible with Ubuntu 18.04

[jameslamb]

Description

See #6293 (comment).

R 3.6 CI jobs running on ubuntu:18.04 started failing recently with this error:

Run r-lib/actions/setup-pandoc@v2
  with:
    pandoc-version: [2](https://github.com/microsoft/LightGBM/actions/runs/7745434225/job/21139413173?pr=6293#step:7:2).19.2
  env:
    _R_CHECK_SYSTEM_CLOCK_: 0
    _R_CHECK_CRAN_INCOMING_REMOTE_: 0
    _R_CHECK_PKG_SIZES_THRESHOLD_: 100
/__e/node20/bin/node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /__e/node20/bin/node)

Reproducible example

Example recent builds from master:

• r-package (ubuntu-latest, gcc, R 3.6, cmake) (link)
• r-package (ubuntu-latest, clang, R 3.6, cmake) (link)

Additional Comments

I suspect this is because r-lib/actions/setup-pandoc just updated to node 20.

Helpful links:

• checkout@v4 doesnt work inside container action actions/checkout#1474
• https://github.com/r-lib/actions/releases