Go back to Ubuntu 20.04 for GitHub tests

Author: cgrandin

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

@@ -25,7 +25,7 @@ jobs:
         config:
           # - {os: windows-latest, r: 'release'}
           # - {os: macOS-latest, r: 'release'}
-          - {os: ubuntu-22.04, r: 'release'}
+          - {os: ubuntu-20.04, r: 'release', rspm: "https://packagemanager.rstudio.com/cran/__linux__/focal/latest"}
           # - {os: ubuntu-20.04, r: 'devel', rspm: "https://packagemanager.rstudio.com/cran/__linux__/focal/latest"}
 
     env: