Change Ubuntu 18.04 to bionic

Author: stefvanbuuren

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

@@ -24,6 +24,7 @@ jobs:
         config:
           - {os: windows-latest, r: 'release'}
           - {os: macOS-latest, r: 'release'}
+          - {os: ubuntu-18.04, r: 'release', rspm: "https://packagemanager.rstudio.com/cran/__linux__/bionic/latest"}
           - {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"}