setup-r-dependencies: set JAVA_HOME on Windows runners from Java 21 env vars

[Copilot]

rJava builds on Windows can fail because JAVA_HOME is not set by default on GitHub-hosted runners. This updates setup-r-dependencies to export JAVA_HOME automatically from the runner’s Java 21 variables when it is missing.

• Action behavior change (Windows only)

  • Added a dedicated Set JAVA_HOME (Windows) step in setup-r-dependencies/action.yaml.
  • When JAVA_HOME is unset, the step maps by runner arch:
    • X64 → JAVA_HOME_21_X64
    • ARM64 → JAVA_HOME_21_AARCH64
  • Leaves pre-set JAVA_HOME untouched.
  • Emits a warning for unexpected Windows architectures.

• Docs update

  • Documented the new automatic Java configuration behavior in setup-r-dependencies/README.md.

- name: Set JAVA_HOME (Windows)
  if: ${{ runner.os == 'Windows' }}
  shell: bash
  run: |
    if [ -z "${JAVA_HOME:-}" ]; then
      arch="${RUNNER_ARCH^^}"
      case "${arch}" in
        X64) echo "JAVA_HOME=${JAVA_HOME_21_X64}" >> $GITHUB_ENV ;;
        ARM64) echo "JAVA_HOME=${JAVA_HOME_21_AARCH64}" >> $GITHUB_ENV ;;
      esac
    fi

• Fixes Set JAVA_HOME on Windows #1099

[gaborcsardi]

Thanks! Why doesn't this belong in setup-r?