Add old archives to be able to install gcc 4.8

Author: DavisVaughan

.github/workflows/R-CMD-check.yaml

@@ -54,8 +54,19 @@ jobs:
           use-public-rspm: true
 
       - name : Install compiler version
+        # We check on this old compiler specifically to support CentOS 7,
+        # which uses this gcc version. RStudio products support CentOS 7 through
+        # June 2024.
+        # https://github.com/r-lib/cpp11/pull/78
+        # https://www.rstudio.com/about/platform-support/
+        # Ubuntu 20.04 technically dropped support for gcc 4.8, so we have to
+        # add old archives back in manually to install it
+        # https://github.com/r-lib/cpp11/pull/279
         if: matrix.config.custom == 'gcc 4.8'
         run: |
+          echo "deb http://dk.archive.ubuntu.com/ubuntu/ xenial main" | sudo tee -a /etc/apt/sources.list
+          echo "deb http://dk.archive.ubuntu.com/ubuntu/ xenial universe" | sudo tee -a /etc/apt/sources.list
+          sudo apt update
           sudo apt-get install -y g++-4.8
           mkdir ~/.R/
           echo $'CXX1X=g++-4.8\nCXX11=g++-4.8' >> ~/.R/Makevars