ci: pin Pandoc to 3.1.9

[alex-chew]

Description

This PR pins the version of Pandoc used to build the reference manual in CI to version 3.1.9.

The last nightly prerelease workflow failed due to a LaTeX error when building the reference manual:

./concat DafnyRef.md | sed -e '/numbered toc/d' -e '/:toc/d' -e '/PDFOMIT/d'  -e 's/<!--PDF NEWPAGE-->/\\newpage/' -e 's/<!--.*-->//' | pandoc --citeproc --toc --syntax-definition="../KDESyntaxDefinition/dafny.xml"  --syntax-definition="../KDESyntaxDefinition/grammar.xml" --highlight-style=../KDESyntaxDefinition/dafny.theme --pdf-engine=xelatex --bibliography=DafnyRef.bib --bibliography=krml250.bib --bibliography=poc.bib --bibliography=paper-full.bib --bibliography=references.bib -H head.tex -B header.tex -V colorlinks=true -t pdf -o DafnyRef.pdf
Error producing PDF.
! Undefined control sequence.
\__tcobox_set_backslash_removed:Nn ...1\tl_set:Ne 
                                                  #1{\exp_last_unbraced:NV \...
l.2522 \DeclareTotalTCBox{\tcboxverb}

make: *** [all] Error 43

That failing build used the latest version of Pandoc, 3.1.11.1, whereas the last successful build used version 3.1.9.

How has this been tested?

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