Use ubuntu 20.04 for nightly GA (#3770)

Task/Issue URL: https://app.asana.com/0/488551667048375/1205793949269201/f

### Description
Uses ubuntu-20.04 instead of latest to avoid issues on ci

More info: actions/runner-images#6709

### Steps to test this PR

_Feature 1_
- [ ]
- [ ]

### UI changes
| Before  | After |
| ------ | ----- |
!(Upload before screenshot)|(Upload after screenshot)|

.github/workflows/nightly.yml

@@ -12,7 +12,7 @@ concurrency:
 jobs:
   code_formatting:
     name: Code Formatting
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 #https://github.com/actions/runner-images/issues/6709
 
     steps:
       - name: Checkout repository
@@ -33,7 +33,7 @@ jobs:
 
   unit_tests:
     name: Unit tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 #https://github.com/actions/runner-images/issues/6709
 
     steps:
       - name: Checkout repository
@@ -65,7 +65,7 @@ jobs:
 
   lint:
     name: Lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 #https://github.com/actions/runner-images/issues/6709
 
     steps:
       - name: Checkout repository
@@ -101,7 +101,7 @@ jobs:
           path: lint-report.zip
 
   android_tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 #https://github.com/actions/runner-images/issues/6709
     name: Android CI checks
 
     steps:
@@ -163,7 +163,7 @@ jobs:
 
   create_task_when_failed:
     name: Create Asana task when workflow failed
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 #https://github.com/actions/runner-images/issues/6709
     needs: [code_formatting, unit_tests, lint, android_tests]
     if: ${{ failure() }}
     steps: