Fix missing bibtex binary and fmtutil warning on cached package restore

[AadilRn]

This fixes the missing citations caused due to the APT package caching action in latex-pages.yml, which skips package installation scripts to speed up workflow runs. Skipping those scripts left the /usr/bin/bibtex symlink missing causing bibtex: not found errors during compilation, rendering all PDF citations as question marks.

In other words, since we are purposefully using caching to skip postinst, the update-alternatives mechanism never generates the symbolic link that points bibtex to bibtex.original.

To resolve, commands were added which links /usr/bin/bibtex to /usr/bin/bibtex.original (or /usr/bin/bibtex to /usr/bin/bibtexu) so the Makefile can compile references from refs/References.bib.

The file also runs sudo fmtutil-sys --byfmt pdflatex upfront to prevent pdflatex from falling back to user mode and printing a fmtutil warning banner. The command pre-generates the format at a system level and stores the binary files in a globally readable directory /var/lib/texmf/, leaving the toolchain in a predictable state.

[CSchank]

@smiths I asked the other team who was complaining about a BibTeX issue to test this out and get back to us. Hopefully we will hear back soon!

[smiths]

Great idea @CSchank. Hopefully it works for them.