added ubuntu 18.04 precompilation os

Author: AlexR2D2

.github/workflows/precompile.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ["ubuntu-20.04", "windows-2019", "macos-11"]
+        os: ["ubuntu-18.04", "ubuntu-20.04", "windows-2019", "macos-11"]
         elixir: ["1.14"]
         otp: ["25", "23"]
         exclude: