ENH: Switch Github Actions Linux environment

Switch Github Actions Linux environment version to `ubuntu-20.04`.

`ubuntu-18.04` is being deprecated and support will end by 12/1/2022:
actions/runner-images#6002

Related to recent warnings in the Azure Pipelines Linux environment
being used in the main ITK repository:
InsightSoftwareConsortium/ITK@497c3a7

Author: jhlegarreta

.github/workflows/build.yml

@@ -9,7 +9,7 @@ env:
 
 jobs:
   build-publish-pdf:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2