fix: remove apt-get upgrade from pdf-prep-force to fix CI snap failure

[izmalk]

Summary

Removes the apt-get upgrade -y line from the pdf-prep-force target in docs/Makefile. This line was causing CI failures on the PDF dependencies installation step.

Root cause

apt-get upgrade -y upgrades all system packages, not just the PDF-related ones. On the GitHub Actions ubuntu-latest runner, this pulls in a new version of the firefox package. In Ubuntu 24.04, firefox is a transitional package that depends on the firefox snap, and snapd doesn't work properly in the containerized CI environment — hence:

Errors were encountered while processing:
 /tmp/apt-dpkg-install-Z28U0P/02-firefox_1%3a1snap1-0ubuntu5_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

This is an environmental failure: whenever a new firefox snap version is published in the Ubuntu archive, apt-get upgrade tries (and fails) to install it.

The fix

The apt-get upgrade -y line is unnecessary for PDF generation — you only need to install the PDF packages, not upgrade the entire system. Removing it makes CI deterministic and avoids snap-related failures.

 pdf-prep-force:
 	apt-get update
-	apt-get upgrade -y
 	apt-get install --no-install-recommends -y $(DOCS_PDFPACKAGES) \

[medubelko]

Seems reasonable to me. @izmalk How did you test this locally? I'd like to give it a try.

[izmalk]

@medubelko I have encountered it in my own Ulwazi repo, where we have the same problem - https://github.com/canonical/ulwazi/actions/runs/29955798613/job/89046899743#step:3:739
I tried it locally (just running the PDF build test was enough for me) and got the same error.
The fix has been tested both locally and in CI - https://github.com/canonical/ulwazi/actions/runs/29957695270/job/89050792575?pr=130#step:3:1