CLAUDE: CI image tianon/latex -> blang/latex

tianon/latex is Debian stretch, removed from deb.debian.org, so the apt update that every
job begins with returns 404 and the job dies before pdflatex runs. blang/latex is Ubuntu
xenial on archive.ubuntu.com and still resolves. Same change already made by hand in
elphelint/afx255-dpcso1. Old line kept as a comment.

[skip ci]

.gitlab-ci.yml

 build:
   tags:
     - latex
-  image: tianon/latex
+#  image: tianon/latex
+  image: blang/latex
   script:
     - which bundledoc
     - time pdflatex main.tex