Update CI to use Ubuntu 20.

Author: mhoffman219

.github/workflows/cookiecutter.yml

@@ -12,7 +12,7 @@ on:
       - cookiecutter
 jobs:
   cookiecutter:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v1
     # Remove apt repos that are known to break from time to time

.github/workflows/main.yml

@@ -12,7 +12,7 @@ on: [push]
 jobs:
   build:
 
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v1