Change Github runner to Ubuntu 20.04

Author: spinicist

.github/workflows/build.yml

@@ -15,8 +15,8 @@ jobs:
         config:
         - {
           name: "Ubuntu GCC", artifact: "riesling-linux.tar.gz",
-          os: ubuntu-22.04,
-          cc: "gcc-11", cxx: "g++-11"
+          os: ubuntu-20.04,
+          cc: "gcc-10", cxx: "g++-10"
         }
         - {
           name: "macOS", artifact: "riesling-macos.tar.gz",
@@ -123,7 +123,7 @@ jobs:
         config:
         - {
           name: "Ubuntu GCC", artifact: "riesling-linux.tar.gz",
-          os: ubuntu-18.04
+          os: ubuntu-20.04
         }
         - {
           name: "macOS", artifact: "riesling-macos.tar.gz",