Pin Github Actions to ubuntu-20.04 (close snowplow#858)

Author: Paul Boocock

.github/workflows/build.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     env:
       SAUCE_USERNAME: ${{ secrets.SAUCE_USERNAME }}

.github/workflows/deploy_core.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   deploy:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     defaults:
       run:

.github/workflows/deploy_tracker.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   deploy:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     env:
       SAUCE_USERNAME: ${{ secrets.SAUCE_USERNAME }}

.github/workflows/snyk.yml

@@ -4,7 +4,7 @@ on:
     branches: [ master ]
 jobs:
   security:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Use Node.js 12