Update Linux builds to use Ubuntu 20.04.

Author: elliottslaughter

Dockerfile.ubuntu

@@ -1,8 +1,8 @@
-ARG release=18.04
+ARG release=20.04
 
 FROM ubuntu:$release
 
-ARG version=6.0
+ARG version=19.0
 ARG triple=unknown-unknown-unknown
 ARG threads=4
 

README.md

@@ -9,7 +9,7 @@ Some details about the binaries and how they're built:
   * Included LLVM backends: all
   * LLVM assertions: disabled
   * LLVM build configuration: release
-  * Host OS: Windows, Mac, Linux (Ubuntu 18.04)
+  * Host OS: Windows, Mac, Linux (Ubuntu 20.04)
   * Host Architecture:
       * Windows: x86_64
       * macOS: x86_64 and ARM (M1)

main.sh

@@ -20,7 +20,7 @@ fi
 if [[ $(uname) = Linux ]]; then
     # Build in a Docker container to ensure we minimize dependencies.
     export distro=ubuntu
-    export release=18.04
+    export release=20.04
     ./docker_build.sh
 else
     ./build.sh