Run CI on Ubuntu 20.04 (#16)

Signed-off-by: karthik2804 <karthik.ganeshram@fermyon.com>

Author: karthik2804

.github/workflows/build.yml

@@ -20,8 +20,8 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - { target: "x86_64-unknown-linux-gnu", os: "ubuntu-latest", arch: "amd64", extension: "" }
-          - { target: "aarch64-unknown-linux-gnu", os: "ubuntu-latest", arch: "aarch64", extension: "" }
+          - { target: "x86_64-unknown-linux-gnu", os: "ubuntu-20.04", arch: "amd64", extension: "" }
+          - { target: "aarch64-unknown-linux-gnu", os: "ubuntu-20.04", arch: "aarch64", extension: "" }
           - { target: "x86_64-apple-darwin", os: "macos-latest", arch: "amd64", extension: "" }
           - { target: "aarch64-apple-darwin", os: "macos-latest", arch: "aarch64", extension: "" }
           - { target: "x86_64-pc-windows-msvc", os: "windows-latest", arch: "amd64", extension: ".exe" }