ci: dependencies issue with package 'usethis'

- Add CRAN mirror

Refs #86

.gitlab-ci.yml

@@ -19,7 +19,7 @@ default:
   script:
     - cd ..
     - echo "setwd(\"$(pwd)\")" > .Rprofile
-    - R -e 'utils::install.packages("usethis")'
+    - R -e 'utils::install.packages("usethis", repo = "https://cloud.r-project.org/")'
     - R -e 'remotes::install_deps(dep = T)'
     - R CMD build airgr
     - rename "s/airGR/${R_VERSION}.airGR/" airGR_*.tar.gz