fix(ci): LaTeX errors found:

! LaTeX Error: File `epstopdf-base.sty' not found

Which is due to error:
$ tlmgr update --self && tlmgr install ec epstopdf-pkg epstopdf-base amsmath
tlmgr: Local TeX Live (2021) is older than remote repository (2023).
Cross release updates are only supported with
  update-tlmgr-latest(.sh/.exe) --update

Refs #184

.gitlab-ci.yml

@@ -62,7 +62,8 @@ before_script:
 .check:
   stage: check
   script:
-  - tlmgr update --self && tlmgr install ec epstopdf-pkg epstopdf-base amsmath
+  - R -q -e 'tinytex::tlmgr("option repository https://ftp.tu-chemnitz.de/pub/tug/historic/systems/texlive/2021/tlnet-final")'
+  - tlmgr update --self && tlmgr install ec epstopdf-pkg amsmath
   - R -q -e 'remotes::update_packages("rcmdcheck")'
   - R -q -e 'rcmdcheck::rcmdcheck(args = c("--as-cran"), error_on = "warning")'