build: fix 'gifski' package installation

Cannot compile latest version (1.32.0) of 'gifski' package, error:
make: *** [Makevars:14: myrustlib/target/release/libmyrustlib.a] Error 2

Fixed by using the "remotes" package to compile the older stable version (1.12.0)

.gitlab-ci.yml

@@ -10,7 +10,7 @@ pages:
       Rscript -e "install.packages('rvest')"
       #session 3
       Rscript -e "install.packages('gganimate')"
-      Rscript -e "install.packages('gifski')"
+      Rscript -e "remotes::install_version('gifski', '1.12.0')"
       Rscript -e "install.packages('openxlsx')"
       #session 4
       Rscript -e "install.packages(c('ghibli', 'nycflights13','viridis','ggrepel'))"