Fix public PDF cover gate on Linux

Author: profvjreddi

.github/workflows/book-build-container.yml

@@ -618,9 +618,9 @@ jobs:
           # while the public downloadable PDF needs the designed cover. Keep
           # the canonical Volume I PDF config as the publisher-safe default
           # and alter only this disposable per-job config for public releases.
-          if [[ "${{ inputs.include_pdf_cover }}" == "true" \
-             && "${{ matrix.volume }}" == "vol1" \
-             && "${{ matrix.format_name }}" == "PDF" ]]; then
+          if [ "${{ inputs.include_pdf_cover }}" = "true" ] \
+             && [ "${{ matrix.volume }}" = "vol1" ] \
+             && [ "${{ matrix.format_name }}" = "PDF" ]; then
             sed -i 's/^    coverpage: false$/    coverpage: true/' _quarto.yml
             grep -q '^    coverpage: true$' _quarto.yml
             echo "📘 Enabled the Volume I cover page for the public PDF artifact"