update ci os to ubuntu 20.04

Author: refraction-ray

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
     name: test
     strategy:
       matrix:
-        os: [ubuntu-18.04, macos-latest] # macos-latest disabled to save quota
+        os: [ubuntu-20.04, macos-latest] # macos-latest disabled to save quota
         python-version: [3.8]
       fail-fast: false
     steps:

.github/workflows/nightly_release.yml

@@ -7,7 +7,7 @@ on:
     - cron: "0 12 * * *"
 jobs:
   tests:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     name: test
     strategy:
       fail-fast: false